Take your compass into a gear shop and compare it with one of their new ones. I did that when I suspected mine was out, and it was, by about 45 degrees, so I bought the new one.
I commented on this while my purchase was being rung up (the assistant was friendly, and spoke my language, both rare things to find in a London area gear shop these days). I was asked, had I put it next to a GPS or a mobile phone? Well, yes, I had, I kept them all in the same small pocket on the top of my daysack! Not any more I don't, they're kept well apart now.