My Paypal was hacked into recently - nothing much really, 2 similar subscriptions to an online fantasy football thing, but it had to be suspended and details changed and I had to complete a printed form swearing that the transactions concerned weren't mine. (And in between them going through and being recredited I lost a couple of quid on the exchange rate!)
I found out when the entries appeared on my egg card statement online; if it had been a postal statement I wouldn't have found out for longer, and there might have been more. So using an on-line credit card helps keep track. (I only use Paypal for ebay normally so these jumped out at me.) And change your password regularly.