The best way is to dissassemble the whole tower and look at the mount. I don't think it's the bearing in the mount. If you open your engine bay on both shock towers you'll see a dust cap; remove it see if the bearings in the mount are lubed.
I've seen very seasoned mechanics drive one of the front wheels hard on an elavated bump in the road with one mechanic on the ground listening and the other listening from the top (motor). These guys were very skilled at pin-pointing noises and resolving the noise on the first part they replaced.
The best thing is to get under there and start measuring everything and banging around (carefully). Look for obvious things.
Also, what type of noise is it? Clicking? metal on metal? tap? thud? etc. Can you recreate the noise? Does it happen when going over a hole in the road or only bumps?
Are all the other bushings down there in good shape? (tie rods, etc)
The mechanic's answer would piss me off and I'd probably start arguing with him until he issued me a refund for any of his labor. I would pay the portion of the $750 for parts, but none for his labor because frankly an answer like his/hers is unacceptable. And I would refuse to pay for his shoddy "guess work." Although, your car may have needed control bushings anyway.
Also, don't rule out the fact that he installed the bushings incorrectly. Was it making the noise prior to the replacement?
