One part of being a TA that drives me nuts, is that you never really know what’s going on during class time. Sure, you can go and sit through every class to find out. But, what if there’s more than one class session? Do you sit in on all of them? And really, do you have the time to sit in on 3+ hours of class each week? Probably not.
Instead, you have to rely on two sources: the instructor/professor and the students. Professors/instructors usually don’t have the time to give you a complete breakdown of what they cover or to fully cover all the details in class. And you may or may not see them between their last class before you have your lab. The problem with the students, is that they are notably unreliable in what they remember about the class. On top of that, good luck getting enough words out of them to learn what they covered. And really, how do you interpret “sort of.”
I find this problem really frustrating, as there really isn’t a great solution. The best you can usually do is: a) try really hard to get the students to keep you up to date and b) design your labs such that the you don’t need to know what was covered in class. A is easier to do than B, as you may not have much control over what B contains.
If you know what an upcoming lab is about, and you meet weekly with the instructor, use the time to ask what they hope to have covered in class and what they don’t think they’ll get to. This way, you can start to think about what material the students will need to know in order to do the lab. And from that list, what material they might not have seen in class before arriving in your lab. Then, plan to maybe quickly re-cover what they were shown in class and spend more time explaining the new concepts to them.
If you can’t do the above, then try to find a few students who will speak up and give you a general idea of what happened in class. Or cover everything in the same detail. If you want, you can give the students a chance to say “we’ve see this already” and then skip ahead to the next bit.