Here’s how you can rule out sleeping with someone:
1. She’s hammered.
2. She seems unsure if she wants to (you should never have to talk anyone into it).
3. She’s passed out.
4. It seems like there’s any other reason she might regret it in the morning. (Even if it’s not rape, do you really want to be someone’s morning-after regret, when instead they can remember you as a total gentleman?)
P.S. My he/she conventions here are in no way an attempt to shut out gay men from the conversation — these are notes to my son (who happens to be heterosexual) and other heterosexual sons. To gay sons out there, just pretend I said he, because this goes for you, too!