The official answer, from a TFL droid I just spoke to, is "You can't. We have no way of checking whether you owe the charge or not; we just get the penalty charge notices a couple of days afterwards."
What a shame the technology doesn't exist to allow this to be able to happen. Oh hang on, it does, it would be an utter piece of cake to do this. Better still, if you're in any doubt, you pay it, and if your plate doesn't appear on the database, they don't take your payment.
Except, oh hang on, they'd lose loads of money from all the people who'd rather pay it to be on the safe side than risk the £50 fine.
This really sucks.
I wonder how much they make every day from people whose plates don't appear. I'd bet it's many tens or even hundreds of people.