The way to have better idea if they lie and just want sex, is not to sleep with them until you dated them sufficiently.

You can't tell me they dated you regularly for 2-3 months, called you daily, went places with you regularly, became friends, took you out, did romantic things, had activities, then you had sex and it turned out the only thing they wanted was sex and they didn't want to date? It's very unlikely. Sex probably took place way before you two dated and became friends. So get to know them first and more than few days

I went to bed with men too soon in my younger years too. Didn't know better. Are you very young?