How can I evaluate a used road bike i intend to buy?
Check the bike is the right size for you

If the bike is still on sale commercially, check what the new price is. You can find out about many brands/models from bikepedia.com 

Find out what components (gears and brakes) it has and what they'd cost new (by Googling)

Check for any hairline cracks in the frame, especially around the head tube, bottom bracket and the top of the seat tube. You shouldn't ride a bike with this kind of damage, let alone buy it.

Check the rims for signs of wear - for example a pitted or deeply scratched surface, or concave feel indicates significant use.

Check the cassette for signs of wear - the teeth should be symmetrical, not sloping more sharply on one side than the other. Scroll down Sheldon Brown's article on chains for more detail and pictures. 

Items like saddles, pedals and bartape can easily be replaced, but you should know in advance what this might cost.

Also see this answer on evaluating a road bike.