The lower ball joint typically supports the weight. So, even if it is worn the weight of the vehicle keeps the ball pressed tightly in its socket. There probably is a specified amount of play that is acceptable and the alignment shops may or may not know what that is or believe it.
And, I have seen a tool that measured play in a ball joint although it was a long time ago. A guy I went to college with had a 60's Mercury Comet that he dropped off at the local Kmart to have the advertised special ball joints installed. They called him up and told him the ball joints were within spec. So, he invited me to go with him and check. The mechanic had a tool attached to the car that did indeed measure the play. On this particular car and I think all Ford cars back then with a similar suspension, the allowed play was 0.250" or a quarter of an inch. Hard to believe but it's true. I had a Mustang back then it drove the alignment guys nuts when they told me it needed new ball joints and I told them it was within factory specs. They would pull out the chart and sure enough, it said 0.250". Then they would say, "Well, they need replacement anyway." Eventually, I replace them just to shut up the alignment guys. The new ball joints were made tighter than the Ford joints.