How do I make it for that the camera can't look backwards?

Hello, I'm scripting a game which will be locked first player. I was wondering how to make it for the camera can't look backwards, and you have to turn your entire character around to do so. I just need a way to detect when the camera moves lets say 90 degrees left or right from the a certain point. I'll take any advice because I've been trying to crack this for 2 days straight. Thanks!