Introduction:
Drug use has been intricately bound to the history of human kind and has had an equally substantial impact on the development of our diverse cultures. For the longest time there were only a limited set of psychoactive substances available as most were directly sourced from nature, or reduced from sources found in nature. Over the last hundred or so years the understanding and variety of these compounds has exploded, resulting in a greater level of confusion when it comes to their individial applications, effects, and secondary or "side" effects.
In the past regulation was a much looser concept, and the choice of when and what a person needed to consume was left to that individual or their guardian. Eventually the minimally regulated drug trade was found to be a public hazard by the powers that be, and the availability and understanding of these tools was taken from the hands of many and placed in the hands of few. These events climaxed in 1971 when Richard Nixon declared drug abuse to be Public Enemy Number One officially kicking off the War on Drugs, although the policies he expanded upon had been in effect in the US since about 1914.
In the sixties when the counterculture was also kicking into full swing, individuals had access to various psychotropics through chemical company catalogs. They could order substances such as LSD and mescaline derivatives by mail order. With the recent arrival of the internet, block-chain based currencies, and encrypted communications, this catalogue style drug market has been reborn in a modern format.
Now that the availability of psychotropic agents has been removed from the power of the few and placed back in the hands of the people, this brings back a few issues, along with some new ones resulting from this period of disconnect. With the amount of new substances and analogs available on the market today it is increasingly difficult and unsafe to identify a substance through purely olfactory means. Since the mid-to-late 90s there has been an increasing trend among unsavory drug dealers to "cut" pure MDMA with cheaper substances to increase the profitability of their product.
Aside from watering down pure products there has been an increase in entirely different drugs being sold in place of others. Umbrella terms such as acid and Molly have resulted in a plethora of substances being distributed under these titles. One of many analogs from a broad family of drugs known as phenethylamines are oven placed on blotter papers and sold in place of LSD due to their significantly cheaper price margin and high availability. In the case of MDMA, cathinone substitutes are often distributed in its place. Although these chemicals are usually distributed as "Molly" or "Mandy", these chemicals are not truly MDMA or even MDA, but synthetic alkaloids which have been modeled off of cathinone, the active ingredient of the khat plant, commonly used in East Africa and South Arabia in a manner similar to the use of coca leaves in South America.
Substance Identification with Test Kits:
These are just a couple of the many reasons supporting the idea of any responsible person using drugs, to first use the appropriate drug testing fluids. Commonly referred to as "test kits", these fluids can be used to identify the chemical nature of a given sample. Though they can't give you a perfect purity level, they can be used to spot adulterants and substituted drugs.
These fluids work by reacting with the present sample chemical in a manner specific to that chemical or it's class. The fluid leaves the dropper bottle clear and reacts with the sample in a unique fashion which causes the fluid to change color over a period of time. Different qualities such as chemical structure and general concentration of a given substance can be observed by paying attention to the colors produced in the reaction as time elapses, and the vigour of the reaction.
Adulterants can be identified by the presence of an inconsistent or mixed reaction from a given substance. For example, MDMA, MDE, and MDA will react first by turning a light purple, and then slowly to darker shades of purple before turning black, when tested with Marquis fluid. When Methamphetamine or Amphetamine reacts with Marquis fluid it turns from a light to deep orange. When MDMA or a close variant, commonly referred to as an MDxx, reacts in the presence of Amphetamine/Methamphetamine it produces a red tone as the purple and orange reactions of the present chemicals blend. Marquis reaction chart courtesy of TestKitPlus.com. 
Now that we have a loose idea of how testing works, let's check out the step by step process of testing as written by my good friends over at DanceSafe.org!
"Never have more than one reagent bottle open at a time. If you mix up the caps and put the wrong cap on the wrong reagent bottle, this may cross-contaminate the reagents and ruin them. Be sure to perform the tests in a well-lit location. Be careful when using the reagents, as they contain chemicals that can damage skin (we recommend wearing latex gloves). Keep the solutions far from your eyes and mouth. Also, make sure to clean up completely after each test.
Scrape a tiny bit of your pill or powder onto a large, white ceramic plate. Use just enough powder to see on the plate.
Take the reagent bottle out of the plastic safety container. Remove the cap and turn the bottle upside-down a couple inches over the powder . Squeeze one drop out of the bottle onto the powder. Be careful to not let the dropper bottle touch your powder or you will contaminate and ruin the rest of the reagent. Replace the cap.
Observe the color change right away. Use the corresponding columns of the color chart included with your kit to evaluate your test.
If the reagent either does not change any color during the first 30 seconds or if it produces some other color-change sequence, then the pill definitely does not contain any of the substances listed on the kits corresponding chart. It may also be highly adulterated with one or more unknown substances. You can use other reagents on new samples to look for the presence of other substances. Wash the testing surfaces with soap and water as soon as possible."-
Source URL: https://dancesafe.org/testing-kit-instructions/#
Here is an informative chart designed by Dance Safe depicting the various color reactions. 
For testing for the presence of LSD and other hallucinogenic indole compounds the Ehlrich test kit is the best bet. 
Closing Statement:
This information is not made available in an attempt to condone illegal activities, but only to attempt to reduce harm on individuals who choose to use substances. The best way to limit and reverse the damage caused to society by the criminalisation of substances is to use your power of free speech to spread word of harm reduction and also to stand up for those people whose lives have been made illegal. Many people who use drugs are forced into desperate situations not by the use of the substance itself but by the damnation placed on them by Society as a whole or their family units. Compassion for our fellow humans is something we as a species need to work on if we hope to continue evolving into the distant future. Thank you deeply for checking out my post! Comment if there are any topics you would enjoy reading about :)
Pertinent Links:
https://dancesafe.org
http://bunkpolice.com
http://testkitplus.com
https://www.erowid.org
https://www.maps.org
https://pillreports.net
http://drugabuse.com/library/history-of-drug-abuse/
https://www.mdma.net/alexander-shulgin/pihkal-tihkal.html