I can send a signed message using the standard client, but if I see a signed message in, say, a forum, how can I verify it?
I understand this hasn't yet been implemented in the GUI for the standard client.
verifymessage [address] [signature] [message]
This feature has also been included in the GUI of the Armory client:
To Verify a Signature Block with Armory (works fine in offline mode):
Open Armory, go to the menu, "Tools"-->"Message Signing" Click on "Import Signature Block" at the bottom of the screen. Paste the signature block, and click "Okay" Address will be checked against public key, public key checked against signature for the given message
None of this requires the blockchain, so if you are on a system that couldn't normally run Armory, you only need to run with the "--noblockchain" option. This allows you to verify Armory signature blocks without even having an Armory wallet! If you're intrigued, try this one: (get Armory if necessary)
You can also do it online. For example here: