A good rule of thumb is that when everyone wants in it is more than likely a time to get out. As far as bitcoin and the cryptos go, I would imagine that monitoring the % of posts here at Steemit that are devoted to the cryptos could at least give some clues. I would certainly not be buying right now as a very high % of trending posts are dedicated to cryptos. Wait until people are saying things like "what happened?" or "I bought bitcoin at $2400 and now I'm down 30%, what should I do now?" ...things like that. :-) There are actually ways to measure sentiment of investors in the tech stocks. Let's look at TSLA since you mentioned it. It is now trading well off the highs and this now means that anyone who paid say $320+ is in a position where they are "hoping" to at least get even on their trade. This means that if TSLA stock trades lower than the $290 low that was set in early May those 'hopefuls" will likely abandon ship. That would lead to price targets in the 260-250 area as outlined on this chart.
The bottom line on how most stocks are priced is sentiment. Price is a function of what anyone is willing to pay at any given time based on their own perceptions. Same with bitcoin, same with how much you are willing to pay for a cup of coffee at Starbux. if price is too high based on your own perception of value you will simply find an alternative or avoid altogether. Buy only at times of fear...sell at times of euphoria. Luckily for traders, investor sentiment can be tracked very easily in the stock market. Maybe not so easily in teh cryptos. But based on this tracking fund for bitcoin that trades on the U.S. equity exchanges...this is EXTREME euphoria. People wanted in at ANY price today. :-)
I'll be posting about various trade opps in the equity markets at my Steemit blog. Hope you will stop by as I will always provide evidence as to whether things are a buy or sell based on what the "sentiment" indicators are saying.