"No" isn't enough
There is a glut of media out there that clearly draw the line of consent at the word "no". I was taught growing up that if someone says "no" that any further advances could be considered mildly as annoying, or as not-so-mildly as sexual assault. The problem with this, in my experience, is that it puts pressure on my partner to speak up, and some people are not up to the task. Regardless of gender it takes a strong person to look someone with more privilege than them in the eye and say "no".
We should be empowering our youth to aspire to this strength and know they are well within their rights to say no to any sexual act they do not consent to. This may seem obvious but I think our society (and it's youth) just isn't aware about what sexual assault really is and can be.
They have to say "yes"
Questioning what your partner is into is just as much about asking yourself "are they saying yes" as "are they saying no". If a person says "no", they're clearly drawing the line. But if your partner is intimidated by their circumstance, they might be unable to verbally object. It then becomes your personal responsibility to pick up on this and differ it from a partner who is otherwise just quiet. It's not enough to wait for them to say "no" with their mouth. If they are not saying "yes" with their verbal or body language you do not have the consent of that individual.
Some aspects of body language should arguably be ignored. Erection of the genitalia and breasts for example, as arousal does not signify cognitive mental consent so much as a physical and animalistic instinct.
So, a few questions to ask yourself;
Are they smiling or do they look distracted?
Do they look you in the eye or do they avoid eye contact?
Are they intoxicated?
Are they talking, engaged in the conversation?
(Additions to the list are welcome)
As a note I think gender is a spectrum between these two types of sexual energy. Male/female, Dom/Sub, Daddy/Baby, call them whatever you will. Beyond the physical aspects of the sexes, 'male' projections of sexual energy feel to be by nature more dominant and imposing in their structural manifestations than that of the 'female'. That male energy just wants to get the job done. They are often cruder as a result, and are more likely to place parts of their designs on top of those of others. (The proverbial glass ceiling?) If you trust someone, listen for feedback to aspects of your designs to which you might be blind to from inside your glass house.