How to tell if u-joints are bad?

My car has always made this "clunking" sound coming from directly under the car either when shifting rather hard or cruising in a high gear at low rpms. I thought it was my stock T-5 going bad, so I swapped in a brand new TKO-500 and it still makes the same noise. Its nothing serious, but it does come to my attention. Otherwise I would not be posting about it. I have an FMS aluminum drive shaft that has about 10,000 miles on it. I think it came with new u-joints when I bought it new a couple years ago. Could my u-joints already be going south? I did not notice the "clunking" sound with my stock drive shaft (at least I don't remember hearing it). I think the noise came shortly after I installed the aluminum drive shaft.

Zero_chance said:

Yeah like was already said, if theres a vibration at highway speeds along with the clunk then its almost definitely your U joints. Usually the rear goes bad first but if you have the money you might as well replace them both theyre only like 20 dollars and it means you wont have to yank the driveshaft out again. You should check your tranny crossmember too. One of the bushings in mine was pretty much toast and it used to clunk around back when I bought the car.

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There are is no vibration at all on the highway no matter what speed I'm going. My x-member is brand new as I bought a new one when I did my TKO a couple weeks ago.