I have started using a smallish, maybe 4" whetstone. I had been using a credit/gift card, but I found that it was so thin that it could easily track at an angle and be misleading. This whetstone is maybe half and inch wide, so I think it gives me a better indication of where I have flat spots or too much bend. Of course it isn't a fail safe, but it's a good start. I like these other folks' micrometer idea, but I don't have one.