Glad you asked...
You can tell if you are being watched if you constantly see someone following you or at the same place as you all the time. If you are being watched it may be stalking and you may want to contact the authorities.
Then again, this answer might help you...
The easiest way to tell if you're being watched is to look around you. Is there anyone watching you? Another way to tell if someone is watching you is by your gut instinct. Have you ever felt like someone was staring at you and you turned around and there was. Your gut won't steer you wrong.

