Some blessed people out there are lucky enough to be born with the instinct of how to read an audience. I am not one of the people. However, with a lot of hard work (and a lot of failure), I've hammered out a few easy ways to gauge how my audience is vibing with me. If you have eyeballs, then you can do these too.
Looking only at your slides
I know this doesn't seem like a bad thing. I mean, at least they're paying attention, right? Maybe. There are a two potential reasons for their eyes to be glued to the screen. One, you might have a lot of text on your slides and they're just trying to read it all, which means that they most likely aren't listening. It's tempting to put all your info on your slides just in case you forget anything, but slides are a supplement to your speech, not a replacement. Bullet points and images are your best friends. They're easily digestable and aid content retention. Two, the screen is more interesting than you. You need to contribute more than your slides, not the same amount. In the context of humor, this means that the jokes need to come from you, not the screen. You can include a funny picture here or there, but they should never be the punchline.
This is obvious. If people are texting, they are not at all interested in what you're saying. If you find this happening, switch something up to interrupt the pattern. Change the volume, pitch, or tempo of your voice. Walk around the room a little bit. Maybe poll the audience or ask them a question. There are ways to reengage them without calling them out and embarassing everyone.
Talking to each other
Another obvious one, but it can be very challenging to overcome this without, again, calling them out and embarassing them. One trick that has worked for me, is I'll start speaking directly to them. Then, next time they glance up, they'll see your unwavering gaze and know that they've been caught. Another trick is to ask them a question. Don't make it a knowledge test to prove that they weren't listening; ask them a personal question and tie that nugget of information back to your topic to reengage them.
Shifting in their seats
Of course, people move in their seats, but if a lot of people seem to be moving in their seats or doing that jiggly leg thing, then they're either uncomfortable or bored. They're moving to keep themselves awake.
This is wonderful. However, be careful not to succumb drug that is laughter and go off script because they liked something in particular. Trust your speech and stick with your plan. A great example of this is the brilliant Key and Peele sketch where they take pandering to the audience to a whole new level.