If you buy motors from an actual motion control company (industrial automation, etc,) then the manufacturer's data sheet will contain the diagrams, charts, and numbers you're looking for.
If you buy hobby motors, then you're kind-of SOL. One reason hobby gear is cheaper than pro gear is that the manufacturer doesn't have to qualify, measure, and then stand behind particular ratings :-)
If a "6 kV" motor isn't quite as fast as it's stated, and your racing truck places third instead of second, well, that doesn't matter so much.
If a "32 foot pounds" servo motor fails to hold a 30 pound box at a foot's extent, and it falls on a factory worker's head, that matters a lot more...