Well, a normal terrorist would be near impossible to spot, since they are trained to act normal and appear normal. We can't go around checking everyone that looks suspicious since you would have to go through a couple million people before finding a real terrorist.
But there are ways of spotting someone with ill intent. There's a video about an Israeli airport that has never been hijacked or bombed before, despite many attempts. Here's the video -
This airport focuses more on the humans that attack then the weapons getting through. There have normal metal detectors and scanners, but their guards are trained to read the body language of people and notice when there is a problem. They label the person of interest without the person knowing, and eventually quarantine and investigate. This method has been successful for decades now, and despite being in the middle of a war, it's still remained the safest airport to date.