How do I see what devices my Snapchat has been on considering I've been hacked?
You can't see the devices where the account has been logged on, because the service doesn't provide this information. If your account has been hacked, access the Snapchat website and reset your password as well as other information you might have attached. This is the only way to overcome issues like these.
Use the following URL to reset the password for the account: https://accounts.snapchat.com/accounts/password_reset_request