First off, I like your signature.
In my experience, 85% or so of last minute resistance is non-verbal. It's rare that I've had a girl actually say "no, stop" or "I'm not ready". This is because usually we'll be making out and I'll be squeezin' her tits or whatever, and then when I go for the vag, a LMR'r will usually just guide my hand back onto her tits.
At this point I wait a few more minutes,
try to get her more turned on, and then try again. Space it out like 5 minutes in between attempts. 5 minutes is a long time when your balls are aching and you're both in the heat of passion, so don't be afraid to get back in there. I'd say about two-thirds of the time, the LMR breaks down after my 2nd or 3rd attempt and the girl wants it too much to activate her anti-slut defenses. It's up to you, but honestly after 3 times of trying to grab her cooch if she's still not feeling it it's probably not going to happen. Same goes for escalating from manual stimulation to full on sex, etc. She'll usually do the LMR non-verbally and it's expected that you try at least a couple more times to prove you're not a shivering spineless wimp.
EDIT: DON'T take a rape charge though. A firm or serious "No" means no, 'kay?