I’ll usually try to make a guess at the answer might be, even if I’m pretty sure I’m confusing a similar looking kanji. I’m a big fan of the ConfusionGuesser userscript to figure out exactly why I was wrong, and help me sort out similar things that are giving me trouble.
If I really have absolutely no idea, I usually opt for “aaaaa” or “あああああああ” with the length of the scream a/あ’s being directly proportional to how frustrated I am with myself for not remembering…