This may sound like terrible advice, but I always find I have to sleep with someone before I know if the attraction is truly there or not.
Of course it depends on your moral views on sex, and your sexual preferences full stop - you may prefer to be in a committed relationship before sex.
If I like someone, I am usually happy to sleep with them on date 2 (provided they are happy with this as well). That usually gives me all I need to know as to whether I want a 3rd date!
I am honest with them and make it clear that even though we are having sex, I am not committing to exclusivity until I feel ready.....
With this approach I have found my lovely FWB who is exactly what I am looking for right now! I was quite attracted to him before we even slept together to be fair, but once we had, the attraction grew tenfold.
Same with all my previous long term partners - my attraction grew significantly once we had slept together. In other cases I have found the sex strangely unfulfilling (almost like I am watching myself from afar) and had no desire to repeat it, so I know the attraction isn't really there, even if the guy is good looking, nice etc. Chemistry is a strange and unpredictable thing!
Obviously it does depend though - I am not suggesting this approach would work for everyone.