Here's a scientific test that she will actually prove how good or lousy you are. It gets a lot harder towards the end - took me about 12-15 minutes.

http://www.faceblind.org/facetests/fgcfmt/fgcfmt_intro.php

My wife, who I've previously branded on here as potentially autistic, frequently loses track of a character in a film if that person changes hairstyle or headwear. Also she needs to meet someone quite a few times before she'll recognise them.

I scored average on the test and was convinced that my wife would come out in the bottom 10-20%. Turns out she scored almost full marks. She has excellent recognition skills but clearly doesn't concentrate on people's faces when she's looking at them or talking to them. I was mortified.