1. He asks for space all the time
If your guy starts telling you he needs alone time, to think and figure things out...then he probably isn't feeling you nor the relationship
2. He takes a long time to reply to your calls/texts
If he used to call you and text you all the time with a response time that was faster than light- but this has changed, watch out.
3. He compares you to other girls
If he compares you to other females and they look so superior in his eyes, then maybe he is looking elsewhere.