OK, thanks to all. I tested all six. I was told about one of them that it was bad, so I marked it with a sad face, just so that I do not forget which was the faulty one.
There were four Fanuc 6L DC servos, one Fanuc 5M DC servo, one Allen-Bradley AC servo, and one Electro-craft servo that is bad.
I tested them with a DC power supply.
What I learned about testing them (from this newsgroup and from practice) is very simple: two of the four pins on a DC servo, if picked correctly, make it spin constantly. Two pins on an AC servo make it make a small rotation and stop. If I switch to another certain pin, the motor turns back a bit.
Pictures of testing can be found here:
Thanks to everyone.
i