The other night I went to the climbing gym, intending to look for a climbing partner once I got there. (My usual gym climbing partner isn't available to climb as often as I want to.) The gym provides a whiteboard where you can write your name if you're looking for a partner.
A woman came up to me and offered to climb. She said she was there with a friend, but the friend didn't know how to belay. This was my only real clue that she herself might be inexperienced as a belayer -- probably the two of them were mainly there to boulder. I checked that she had the card tied to her harness that the gym gives you if you've passed their belay test. I belayed her on a climb, and then she belayed me. The belay device was a grigri.
When I was ready to be lowered after finishing the climb, she initially dropped me about half-way down the wall at very high speed, then suddenly stopped me and lowered me much more slowly the rest of the way. When I got to the bottom, she was very embarrassed and apologetic. She apparently hadn't understood how to control the speed when lowering with the grigri, and had probably just pulled the lever back all the way and let go with her hand on the brake strand. She left immediately after this.
Does anyone have suggestions on how to screen potential belayers in a situation like this, and what to do if they seem inexperienced? The best thing I'm coming up with so far, other than checking for the belay card, is to ask explicitly how experienced they are. (A lot of people seem to ask, "How long have you been climbing?," which is probably a nice neutral way of getting this kind of information.) Then if they say they're inexperienced, I can give them a quick mini-review.