OK, you called it quitsville, packed your stuff, and moved out. You're living single and fancy-free, able to play the heavy metal at 11pm while you dance in your gym socks if you want to. Never been happier, right? Well, actually, wrong: You may be still in love with your ex.
You can tell simply by how much of your time you spend with your ex-partner still on your mind. Keep in mind that this is a function over time. If you were together for fifteen years, it's normal for those memories to come back to you, or your thoughts tend to turn to something associated with them. But after awhile, you should have moved on, unless you still have some feelings for them.
A sign that you're still in love with your ex is if you find yourself imagining a future with them. Unlike memories of the past, fantasies of the future indicate that you still want to be with them.
Don't think that just because you're angry or hurt, that this stops you from having feelings for your ex. You can still be in love with someone even if you're fighting with them, after all. And remember that the opposite of love is not hate; the opposite of love is indifference.
Also don't think that just because your ex acts differently towards you, that your feelings aren't shared. They might still want to be with you, but not know how to say so. Or they might want one part but not the other part - you made a great bed-warmer, but your radical attitude had to go. As the old Woody Allen joke about the man whose wife thought she was a chicken goes, they would take you to the doctor, but they need the eggs.
Feel free to communicate with your ex if there's still enough mutual tolerance in each of you to deal with each other. Even if you don't get back together, there is still such a thing as a friendly break-up. Facing feeling that you don't want to have is the toughest part. We don't want to be as vulnerable as we are. We want to be able to just dust off our heels and take off without any emotional strings attached. But humans aren't built that way. Learning to deal with your feelings and put them in perspective is what the game's all about.