There are many signs to look for when trying to find out if somebody is nervous.
One thing that you may want to look out for is that a person may be very ‘fidgety’ and cannot keep themselves still for decent amount of time. They will always want to find something to take their mind off what is making them nervous. The things they may do are tapping an object like a pen on a table or their body to keep a rhythm. Or they may be pacing up and down a room with the aim to take their mind off whatever is making them nervous.
With the feeling of being nervous. a person may be any of the above, or they can be the complete opposite. an example of this could be that they are sat or stood motionless and just constantly staring with minimum blinking. they may be pale or very red in the face depending on the person and they may start sweating quite a lot for just walking or sitting around.
All these factors may contribute in trying to figure out if somebody is nervous.
practice this at your workplace or when you are in education etc.