You shouldn't necessarily load it up as much as you can- you should load it as you would normally use it.
I prefer truck stops for weighing but they're really convenient for me. CAT Scales is the brand that many use. They have a handy locator on their site:
CAT Scale Locator | CAT Scale
Other folks go to grain places, dumps or even bulk agriculture supply places.
I like the CAT/truck stop scales because you can get individual axle weights easier as there are 2 separate scale pads that you pull onto.