Some of the big breeders will send you paperwork anyways. I know Justin Kobylka and NERD will include paperwork. When you buy from these reputable breeders, you can probably safely assume that it's het. The only reason I would really ask for paperwork is in case I want to sell that snake in the future. I want to prove that I bought that snake from so and so.

I'm not sure you can trust paperwork... You can't prove that the snake is in fact het... It's still sort of a gamble. This is why people say to buy hets from people you trust.