If someone used to love you, there's always going to be a part of him that still cares for you, unless something horrible happened (and sometimes even still).
As far as signs go, it gets pretty blurry because signs he might still like you could just mean he still just wants to be friends. Like a good sign is if he still tries to hang out with you, calls you up, does stuff for you. This stuff doesn't necessarily mean he likes you as more than a friend though, because hanging on to a girl after a relationship is tough and sometimes you really have to struggle to keep a girl as a friend by really trying to do stuff with them (so it could all be about friendship, not more than friendship).
Plus, sometimes when you're around someone you used to love, you randomly and without reason get the same symptoms of being in love, like messing up talking, nervousness, awkward silence, etc.
If he tries to bring up the past relationship that's a really good sign he still has feelings for you. If dude is talking about how much other girls suck still or how good things used to be or how much fun you guys used to have, reminiscing and whatnot, he probably still likes you.
At the very least, he wants to mean it and is trying to put his feelings aside so you can still be friends. When you ask things like this though, where there's a chance the guy's answer might mess up an awesome friendship (and then he'll lose all of you, not just your past relationship) he really has no choice except to take the safe way out and say no he doesn't like you like that anymore. You have to make sure he understands you're totally 100% ok with him admitting if a part of him still feels for you, or he's not going to do it. If you ask him like that, you can probably believe him and you won't need us to read his mind for you.
So yeah, there's no real way to be sure, best bet is to look at the signs and what he says.
One last note is that even if he still has feelings for you, don't punish him for admitting it. A lot of girls don't understand how a guy can like a girl but just stay friends with them (and for real some guys can't). What you have to understand is it's kind of like walking through the mall. We would love more than anything to nail every hot girl we come across, but we don't now do we? We accept not having everything we like, that's just part of being a man.