Prosopagnosia is face-blindness. Sufferers find it difficult to recognise people from faces, and forget if they have seen a face before. Here is an online test for prosopagnosia. The mean score for non-impaired is 71%; I got 68%. If you get below 47% you may have an issue. This isn't one of the usual idiotic tests I link to - it was designed by a team researching the condition - but of course it is not clinically definitive on its own.