Chek-Mate uses a postal type scale that is calibrated to be within 2# @ 200# (or .5# @ 50#), and Marc does measure by AMO standards. Close enough for me.
I have a non-Easton digital scale, and so far it's been on the money.
Something I didn't see mentioned--you can change the draw weight of your bow slightly by raising or lowering the brace height.
I've had a few people over the last 10 years or so tell me that their bow wasn't marked correctly. When I asked, the answer was always the same--a hanging spring type scale. I always suggested they check on a calibrated bow scale. Never had to send one back because the weight was off.
Chad