When in doubt, verify with Blockchain.info or BlockExplorer.com
The blockchain never lies.
Which is how you can tell that I have been paid all the bitcoins:
https://people.xiph.org/~greg/21mbtc.pngThe blockchain itself doesn't lie, but websites with blockchain in their name aren't the blockchain. They're websites and they can lie and have lied in the past.
Seriously, depending on some centralized services is not a good idea. Every Bitcoin client itself has an independent view of the blockchain. Does the transaction in question have a bunch of confirmations (e.g. on the order of 6 per hour since you made it)? if so — it's settled. If the far end says they don't see it then something is broken there or you paid the wrong address, or they're scamming you.