Points About Observing Class
It's important to check whether the art, this particular instructor, this school, etc. seem to fit your expectations. It's also important to confirm for oneself whether committing to attending 2 times or more per week is feasible and sustainable.
Check Your Goals For Observing
The final, most important thing to pick up is, "Is this a place I can see myself coming to a few times a week for the foreseeable future?" This is a general feeling - the total mix of the instructor, the other students, the thing being practiced, and the energy in the room.
Other details to check:
What does aikido practice look like? Does it look like something else you've seen? What's different, then? Is it fast or slow? Is it vigorous or laid back? Does it speak to you?
How do the people look? Do they look like they're having a good time? Are they helping each other? How do they interact?
The unspoken rules, or culture, of the place - does it seem like there is attention and care taken, or is it laissez-faire? What kind of atmosphere does that contribute to?
In aikido the forms and roles are predecided and the practice is of repetition. When observing people practicing that repetition, how does it look?