If it's through eBay. It should say to who and what percentage of the sale will go to charity. If I recall correctly, on my listings it's right above the description. With a link and short description of what the charity does.
There is one stipulation that I don't like though. If they can't get the money to the charity for some reason, they'll choose another that is similar. Probably doesn't happen very often. But if they can't get the money to the charity, they should give you a few days to have it go to someone else.
If it's the seller saying it'll go to charity. You'll have to ask if it doesn't outright say and how they're being truthful.
