I had a situation a short while ago where I was using an infra-red remote and it simply stopped working. I assumed the battery had died or wasn't making contact properly. Back at work I asked a techie if he could test the battery on his ammeter, and he suggested that I just click the remote while looking at it through my cell-phone camera. I did this and it turns out it's working just fine: you can actually see the light blink.
This works with a DLSR too, but only in LiveView. It's a handy alternative to prising the remote casing open and testing the battery with a meter.
I still don't know why it stopped working on the day.