How can we tell if there is a p-trap installed under our basement shower stall?

There should be or you would have sewer gas coming in.

Pour some water down the grate. If you hear water trickling, you have a trap or have someone flush a toilet as you put your face at the grate. Feel no air, have trap, bubbles you have a trap, full air flow and stinks, no trap..

Thanks so much. We've now determined there is one since we can see water below our grate.