I suggest you check your bank account to see what id number was used to set up your account. If it's a personal social security number, then your PTO is probably not 501c3. If it's a tax id number (starts with 2 digits, then a dash, then 7 more numbers), then you'll want to find out to whom that number is registered. Call the IRS at 1-877-829-5500 and ask them to check if that number is registered to your PTO. They won't tell you to whom it is registered, but they will tell you if it is NOT your PTO. If it's not your PTO, then it is probably your school or school district's number. If the number belongs to someone else, then your PTO is probably not 501c3. If the number belongs to your PTO, then the IRS can tell you if you are a registered 501c3. But like mum24 said, if you've never seen any reference to 501c3 like a Form 990 or Form 1023 in your files, then your PTO has probably never filed.
If your PTO has never GROSSED over $25k, then you are fine. You do not need to apply for 501(c)(3) status until you grow to $25k.
By the way, if your bank account is setup with a personal ss# or someone else's tax id number, you should seriously consider getting your own federal tax id number. Download Form SS-4 from
www.irs.gov/forms
to get a free tax id (also called EIN or employer id number). This doesn't make your PTO tax exempt or 501c3, it is just the first step. Once you have your own id number, you should seriously consider changing over your bank account to reference that number so there's no potential for confusion over who owns the money in that account.
Sorry for the ramble...hope it helps.