Talking All About It
Look out for unusual patterns in his behavior when it comes to discussing his ex-girlfriend. If he cannot stop talking about his ex-girlfriend, that indicates that he's hung up on her and not over the relationship. If he mentions his ex all of the time for no apparent reason, she is still on his mind. At the other end of the spectrum, not mentioning his ex at all can also indicate that he's on the rebound. If you speak of the concept of exes in general, and he starts to look morose and melancholy, you may have touched upon a sensitive topic and reminded him of feelings that he hasn't entirely dealt with yet.
Another sign that a guy is a rebounder is if he was the one that was dumped by his ex. If his ex-girlfriend initiated the breakup and he told you an elaborate and heart-wrenching story about the situation, he likely is trying to dull his pain with a temporary new and not-so-serious relationship -- unfortunately, with you.
If a guy has a negative, pessimistic and overall bitter attitude when it comes to romantic relationships, it's probably because he still hasn't resolved his feelings about his last breakup, which means he isn't over it.
Barrelling Down the Runway
When a guy wants to move a relationship too fast, it also can be a sign that he is on the rebound. During this time, men often quickly want to emulate the feelings of commitment they had in their past relationship, often without taking the other person's feelings into consideration.
If you are dating a guy who seems to have small triggers that set him off and make him feel a sad and distant, he likely is on the rebound. Some examples of triggers include romantic love songs, television shows and restaurants. If these triggers cause quick changes in his mood, it's because they are reminding him of memories of his ex, whether they are good memories or bad.
If you try to get emotionally close to a guy and he immediately reacts bizarrely by pulling away, he probably isn't very comfortable with displaying a certain level of intimacy with a girl who isn't his ex; in other words, you're a rebound girlfriend for him.
If a guy is overly enthusiastic about you and the new relationship, it could be a sign that he is overcompensating and trying to convince himself that things are so great to help him get over his ex. If you feel that a guy isn't being genuine in his adoration for you, it's probably because you're his rebound girlfriend.
Inspect the Home
If you visit a guy's home and his bedroom appears to be a shrine for his ex-girlfriend, then it's absolutely clear that he is in no way over her yet. From photographs of them together in happier times to ticket stubs from concerts they attended as a couple, the guy is clearly trying to deal with the ending of a relationship and is in no way ready to be dating someone, specifically you, yet.