If you don't have a scale, ask your dealer.
If your dealer won't throw it on a scale in front of you, I'd walk away. It's probably not a dude you want to be associating with anyway.
You can also pickup something as simple as a hand scale:
http://www.bolinat.com/unique/products/reg/708.jpg
They're not very useful for weighing out small bags, but they're perfect for checking weight! They're usually referred to as "postal scales" and most gas stations will have them. You just have to pay attention. They're under $5.