Just this weekend I helped my son in law put a new tub in. After it was done, we filled the tub to the overflow line and let is set a few hours. That gave us confidence the drain shoe was sealed OK. We then ran water full blast to fill past the overflow. That let us see that the over flow was sealed. Finally, we opened the tub drain. That amount of water with a little head on it would have exposed any problems with the W/O, trap, etc.
The wall is completely exposed, so we have access visually, with flashlight and mirror. to the tub box area. And his young, flexible long arms actually could reach under and "hands on" all the fittings.
We are confident with this test.
It was a fun job, because in essence, the tub was about 60 1/8" long ( americast, with resin up around the flanges) and the rough where the old one piece came out was actually about 59 7/8". Needless to say, the sawzall got a workout on this job!