Just go to your airline's web site. They all post their current baggage policy. It would appear that they are being sticklers about this, too. A couple of weeks ago I had a 51.5 pound bag and they would have charged the fee, but I was able to remove an item and get under the limit. I've also seen them pull out a tape measure and check the size.
If you're elite with a particular airline, almost every fee (heavy baggage, oversize, whatever...) is waived. The only one where they seem to be a stickler is the 75 lb limit.