The first tool, GS Lookup - Snapchat, comes via the well-known Gibson Security firm. There, you enter your username. If your account info was not compromised, you'll see a "You're Safe" message. If, on the other hand, your data was leaked, you'll see your phone number (minus the last two digits) displayed on the Web page.
A second tool, Snapcheck.org, works much the same way, except you can enter either your username or phone number. After submitting the info, you'll see be told whether or not your information was included by the hackers.
Notably, if your data was included in the leak, there's not much you can do about it, save perhaps opening a new Snapchat account. Worse, though, is the fact that such a move won't remove your phone number from the circulating database; although it was not published on the now-down website it was leaked to, the hackers do have that information.
That is a big issue, as many people won't change their cell phone numbers even if they move to a different part of the country. Still, that would be the only way out of the issue. Don't worry about your provider giving you a hassle; point them to the details and they will surely help you.
In addition, it's always good in this sort of breach to change your password. While that information wasn't included in the published database, certainly, if you are one of the many who tend to use passwords like "12345" -- or even if you are not -- you should change it.