The tapping the foot thing is a good idea. I've seen that called out in lessons - perhaps here, possibly in some random youtube video I stumbled on.

Something else that I thought of is how well do you know this drummer? Jamming and playing is a lot like having a conversation, and conversations are better when you know the person better. Conversations with somebody you just met can also be awkward. In a musical context, this means that you know a bit about their playing style so you can anticipate what they may do next...and the first time you play together may have some awkard moments.