I usually get a good many arguments started about rotary table setups. I worked in a large forge die shop, and I still do the setups the way we were shown in that shop. Probably 95% of the time you used a rotary table on a rotary head milling machine, so getting stuff on center was step #1.
The first thing to be pointed out is that the center hole and OD of the table aren't necessarily on the axis of rotation. Easy to check, take the worm out of engagement and pull the table around by hand with an indicator zeroed on the center hole. Just like indicating a part in a four jaw.
If it is on center, that's great. If not, you can eyeball your part on center and lightly clamp while you indicate it in by pulling the table around by hand and tapping it. If you don't have a concentric hole or OD to use an indicator on, a center punch mark and a pump center can be used.
Once the part is on the center of the rotary tables axis, it's a simple matter to center it under the machine spindle by locking the table and rotating the machine spindle and indicating like you would normally.
It all sounds like a lot of extra work, but it only takes a few minutes to do.
Kevin