I am fortunate to do business(been about 15 or more years now) with someone who not only produces top-quality hay(alfalfa, and beardless wheat, which I may try next year), but has an on-farm scale, so sells by the ton, and the proof is right there! I pull onto the scale w/ truck and trailer, they weigh the whole rig, me included!...then load, and pull onto the scale again-difference is what I pay for, at the quoted per ton rate, no matter how many bales I get. This also allows you to get a good average on what each bale weighs.
I believe that most sellers 'overstate' what each bale weighs, because they are well-aware that most people either have NO 'feel' for actual weight, and/or they have no way to CHECK on whether the weight is accurate---as in, being able to weigh bales themselves(talking mostly of two/three string bales; those 'handleable' by an individual.) This is a big reason why I prefer NOT to buy by the bale...though in most cases nowadsys, that's the ONLY way it is offered for sale. I DO have a good sense of 'measurement', whether it be weight(of hay,at least, because I've hefted plenty that I KNEW what they weighed),or other indicators...height, width, length, etc., so, when I buy from anyone other than my longtime trusted alfalfa supplier(above), I will heft a representative bale before paying...and being the old curmudgeon that I am, I may challenge the seller...or take a bale home and weigh it on my hanging scale, and they will hear from me if the weight is too much lighter than 'suggested'.
Margo