I have real trouble telling whether I have seen somebody before (and if so, where).
Often people come up to me and say "hi" and I have to tell them "I'm sorry, I don't know who you are". Sometimes I recognise them a bit, and sometimes I do not recognise them at all but they recognise me!
I get extremely embarrassed about this. Often, I say "I must have been drunk when I met you!" Because I am semi-alcoholic, this is usually plausible. But it is not really the truth (not all the time, anyway!). I know that really I don't need to be drunk for this to happen. Firstly, I don't look at people's faces as closely as others do. Secondly, my eyes dart around so I only get a general impression of the face. Thirdly, when I meet a new person and talk to them my anxiety level goes sky-high and blocks out any chance I would have of concentrating or remembering anything.
Perhaps that is one reason to get a diagnosis. When it happens, I can say: "I'm sorry that I don't remember you, but don't take it personally. I have Asperger Syndrome and it makes it difficult for me to remember faces." That would be nice.