How to know if a known player has genuine feelings for you?

Players are like every other guy, they're not immune to developing feelings for and becoming emotionally attached to one girl. So in that aspect, look for the obvious signs; e.g. wanting to spend most of his free time with you, even if sex is off the table and wanting to share his life with you etc.

However, your question should be "can a player be committed to me even though he has feelings for me". The answer is simple. No! The quintessential player commits to no one in his prime.

since he's your friend, I suggest that instead of making a move on him, or trying to decipher if he has "genuine" feelings for you, you could simply start by discussing relationships with him. If he's a known player, confront him about this, not in a mean "I disapprove what you do" way but try to know if he sees himself in a long, committed, stable and genuine relationship at one point. You could ask him why he does that, and if he doesn't get tired of it sometimes (and by the same way you could try to make him see how a stable and genuine relationship is much more rewarding than what he does).
That way, you will know if he, at least, has ever thought, about being with one girl he would love genuinely

Now I know this doesn't answer your question. But, from my point of view, there really is no way to tell whether someone has genuine feelings for you or not, and this regardless of whether they're a "player". You can only know this by being in a relationship with them and as time passes by. Which is why getting in a relationship is scary and makes one up for hurt. I could be wrong but from the way you asked your question, I had the feeling that because he did like you at some point, you believe he may still have genuine feelings for you. And this is perfectly understandable, yet the only way to be sure of it is to take some risk and thus take the risk to be wrong and disappointed.

But either way, I am not that much a fan of a direct approach, I think it is better to know where people stand regarding relationship firsthand.

Oh geese. Players are so hard to deal with. You just never know if they're telling the truth or not. Him asking you to hook up is kind of a red flag though. Maybe test him by making him wait for sex to see if he really likes you. You don't need to make a big deal about it and let him know what you're doing, but make him wait at least two or three weeks and see if he actually sticks around. That's one good way to test a player.

Judging by my dad, when they turn 50 and realize that women stopped liking them about 15 years earlier so they settle.