How can I tell if this is going to work?

It's not going to work. She has to step back, and realize what she really wants. It might be you, it might be her ex, or none of the above. I know you don't want to let go of the sex, but if you keep in contact with her while she is in this state, someone will get hurt.

As for what's going through her mind. She is creating a safe spot for her to fall on. Having you around makes her feel confident that no matter what happens to her and her ex, she will have someone there. If she really cared for you, she wouldn't string you along her issues with her ex. Itz obvious she is not over him yet. No matter what she says. If she is still bringing up his name, or anything about him, it means she is not over him.

You can tell if its going to work; and its not. She's waving red flags by saying she is scared she'll go back to him and comparing you to him. Keep your distance till she gets over this guy.

First please refrain from using the F word even if you did censor it. It's demeaning.

Second, why would you want to be with a girl who keeps going back to her ex and can't even make up her mind.

I guarantee you she'll play head games with you all the way through.

A ma dude her previous relationship didn't work out good and she is upset because it didn't work out. She goes to you and know you be hitting it so is all good. She sees what you got that her ex doesn't have that's why things work out with ya. She brings up the idea of going out so she don't seem dirty that you be hitting it and you guys ain't going out. She is using you as her rebound. You basically are the one that picks up all the broken pieces of her heart when her ex destroyed her heart. That shit won't work bro. Trust me cause I was with shorty same situation. Shit ain't work she gonna be thinking about the ni99a . She eventually gonna go back to him since you a rebound. Don't put yourself in that situation. Go find another shorty or if you want to keep on hitting it go ahead but don't get Pus*y whooped. Be there but talk to other shorties too cause you ain't even dating her so its not that you be a player. So when she stop talking to you , you already got other shorties there

I think its quite obvious that she knows that she is likely to fuck you around because she doesn't know her own feelings, and good on her for being honest enough to say it straight.