[sugar and spice]

Obviously there are no concrete rules, but one I've noticed is that they tend to get serious about you too quickly. If a guy really wants to get to know you, he will hold back and take things one step at a time. If you're a rebound, he'll treat you like you're just a stand-in for the girlfriend he just got dumped by because he's not used to being alone and he misses the gratification of a relationship. So if he starts saying "I love you," etc. way too early for comfort -- don't take this too seriously.

Also, if he starts comparing you to her -- even if it's in a positive way, i.e., "You are soo much smarter than Jessica" -- that's a major sign that he's not over her yet.