I'd check things out before you enter anything into bitaddress.org...but didn't someone mention code there that sends the information to a server? I wouldn't want anyone with my private keys except myself. I wouldn't do anything online with that sort of data, unless it's for like 0.1BTC...but even 1 BTC I wouldn't do it, it's too risky. People will steal 1BTC without any qualms.

You should be able to verify if a private key points to a public key. It should be easy to do so. Just load bitcoin-qt, wait for the blockchain to sync, and use the importprivkey command to import the private key into the client. bitcoin-qt will find the public key for that private key and then proceed to update the balance. It's not that hard.

Don't use online stuff unless it's heavily password protected, and even then I wouldn't even trust it. I've heard too many stories of BTC going AWOL because someone wanted to test something out.