If all they keep talking about is that someone, then chances are they are using you. But otherwise trust your instincts, if they are with you for you, you will know it.
probably time clears it all
If your a bounce back relationship you will end up dumped and they never really cared about you.
I think you will always know at your heart.
As you go along, time only confirms it.
Some people are obvious about it by the way that they talk about the ex or compare. Others are great at hiding it, or might not even realize what they're doing.
Keep it funky and ask. what the worst you get the truth... Maybe they aren't aware them selves.
Certain key phrases...