Okay, here's another idea...go to
www.irs.gov, and search on Publication 78. It's the online search for registered charities. If you put your PTO name in the search field, you'll find out if it is a registered 501c3. If you get a hit, all that tells you is that it must have its own tax id number. If it has its own tax id number, it could possibly have its own bank account - somewhere. If the account is in the name of the PTO and under the PTO's tax id number, then at least your PTO owns the money. If the account is under someone's personal SS#, you might have a hard time proving it belongs to the PTO even if you can find it.
Wait - here's another idea...call the last fundraising company your PTO used. They should have a tax id number on file for your PTO. If you get the tax id number, then you could try calling the IRS at 1-877-829-5500 and ask if that number belongs to your PTO (you might find out it is actually the school's tax id number). Go branch to branch with the tax id# to the local banks (those within about 5 miles of your school) and talk directly with the branch manager. If you find an account for your PTO, find out who has authority to the account (who is on the signature card).
Who do you think signed the PTO's checks last year? Was there a treasurer? Did your group do any fundraising?
Like I said before, you can start a PTO from scratch, but if there's $$ in the bank, you need to get a hold of it. Eitehr the former president is so burned out that she doesn't want to spend one more minute on PTO - even training you...or she's doing something she's embarrassed/ashamed of and she's trying to avoid you. I'd go knock on her door.