In a binder with all sorts of documentation that came with the boat, I found an almost-totally-faded deviation card. I can't read some of the values. I've never sailed anywhere that I couldn't see where I was going, so even though we use the compass, we don't depend on it. When I wasn't sure, the GPS told me... some day I'll get around to making my own deviation card.
Also don't compasses usually have weak internal magnets that are placed to cancel out the deviation?