It is an interesting topic, particularly once you bring in longer term relationships where there is often an element of presumed 'ongoing' consent to low level touching such as hugs, kissing, over clothes touching etc. This is complex and established over time through trust between couples so really doesn't apply in the sort of case we're talking about.
With a stranger, a young relationship, a one night stand, etc. you're on much thinner ice. You have no real knowledge of them or their 'signals' or wishes or views. It is never going to be defensible to 'assume' with a stranger. The best bits of advice:
1. Communicate - ask, be specific, and continue to ask as you go. You don't have to make it sound like a market research survey, asking can be sexy. If you want to be 'bold' and 'seize the moment', be bold in asking.
2. Never assume that because they have consented to one thing, they are therefore consenting to something else. Kissing is not consent to touching genitals, for example.
3. Consider capacity - if someone is very intoxicated, has significant impairments, or there is a significant power imbalance between you as is often the case with these Weinstein type cases, capacity to consent is an issue. Could this person genuinely and freely say 'no' to you right now?
