A few rules I have (wish I had these earlier, as I have a couple of regrets)
1. Assume the lady is only willing to have sex because she wants money. If that's not enough for you, get out of the game. You might be lucky that she also fancies you, but you'll never know for certain. Even with fully independent ladies, you have to assume you are paying for consent, and be ethically comfortable with that. If you think payment invalidates consent, then take up golf.
2. Nobody who advertises as a teenager. Even 21 and under. I'm glad I don't have to take too much responsibility for crazy shit I did at that age. No doubt, some girls at that age can handle the responsibility, but I'm not taking the chance. On a side note, my worst experiences so far have been with under 25s.
3. If she says no, that means no consent. Stop. Move on. Learn whatever lesson you need to from that.
4. Nobody who can't communicate verbally in English. They are too vulnerable and dependent on 3rd parties for support. Texts can be from anyone.
5. Nobody who shows any sign of fear or sadness.
I never ask for consent, but then I never ask for sex. It just happens. They clearly love me.