Humans have developed certain social behaviors over millions of years in evolution that helped them to survive in the wild, but can now be used in your favor (and against theirs) in the modern world. Here are a few things I've found useful in my research and I use them myself at appropriate times when interacting with other people.
1.) If you think someone is going to give a negative opinion about you in a professional setting, sit next to them. Because of people's unconscious need to protect themselves from harm, they won't provoke someone who is sitting right next to them, and if they have an important criticism it won't be as harsh.
2.) If you want to know if someone is spying on you or checking you out, yawn or check your watch. Cells in the human brain called "mirror neurons" (responsible for empathy and behavior) will cause them to yawn or check the time as well. Just be sure to be looking around to catch them.
3.) I read in the book What Every Body Is Saying (written by a former FBI interrogator) that people are the best at controlling their faces when lying so don't bother looking there if you are trying to spot a lie (unless you are good at spotting micro-expressions). An easier way would be looking at body language:
if they are crossing their arms or legs (protecting the vital organs; a signal of discomfort) and/or pointing their feet away from you. This is because Humans know that when they lie they need to look innocent but their focus on maintaining that facade is less and less intense the further away you get from the head. So if you want to know the truth check their feet.
Be aware if they are touching their face a lot or rubbing other parts of their body such as arms, hands or their lap. This is an unconscious behavior meant to soothe themselves in tense situations.
You should also pay attention to their breathing; if they stop breathing that means you've caught them off guard and activated their "fight or flight" response.
4.) If you know someone is hiding something that they don't want you to find, move around the room and look where they are standing, people who are unaware of this trick will usually stand between the object and the person looking for the object. For example: If you know your wife is hiding, let's say, the remote somewhere and it's in a drawer. she'll stand between you and the drawer. Of course, this isn't the case 100% of the time but be aware.
5.) When a group of people are laughing, everyone looks at the person they like the most.
