After a lineup show other comedians sometimes ask me for "notes" (feedback) on their performance. I usually can't do this in a useful way, this is specially true for shows I'm helping to run such as our own One Mic Stand-up but also applies when I'm just a member of the line-up.
If I'm at a show, I'm almost always there to work. I may be a performer, I may be the MC, or I may be the producer.