On an old 240D that we had, it was very obvious that left front ball joint was bad. Going over rutted and bumpy roads it seemed like the front end was going to fall apart. But then, not knowing how to test, I would shake the wheel while on the ground and it didn't seem to have a problem.
The boot was almost completely gone. I replace the upper with a new one from MBZ. The change was wonderful. Smooth, no slack!
I still have the old one. It is mounted in the upper control arm. I used a buffer wheel and cleaned back to its shiny aluminum original state. It is fun to look at, mess with and show what a ball joint is.
What is really remarkable to me is, yes it is loose in its "socket" but not that loose. I can move it around and find the slack, but it is less than 1/16" max movement. It seems any play will definitely be detected in the right test situation such as csnow above pointed out.
'03 E320 Wagon-Sold
'95 E320 Wagon-Went to Ex
'93 190E 2.6-Wrecked
'91 300E-Went to Ex
'65 911 Coupe (#302580)