How would one assess the condition of a bottle of vintage wine?

Lets get the 'drink it' aswers out of the way first, shall we.

I'm assuming you mean before it's opened and I'm assuming you mean a red?

If you can see the cork, check for veining which occurs if air has gotten into the bottle (this will appear as a line or blotch of red on the cork and would indicate that the integrity of the wine has been jeopardized). If you can see through the glass, check for the presence of sediment at the bottom of the bottle. Check for color, is it starting to turn brown?

Keep in mind that unless you know the history of the wine, (eg. if it was stored properly, etc.) you're really only going to be able to tell once you've opened it and can see, smell and taste the wine to know if it's still good.

The tell tale signs are any discoloration of the cork. I've only seen a few wines that actually were ok that had coloration on the cork. Also if the bottle looks like it is not full enough, or there is any dried wine running on the side of the bottle.

Once opened, you might be able to tell if it has really turned right away by smelling. I would let it breathe for 10 minutes before giving up on most wines though.