How can I verify that my basement drain has a trap? I live in a late 1960's home and I want to install a shower in the basement. There is what looks to be a 2.5 or 3 inch cast iron hub with a cap on it for a drain. It appears as though the basement was built with the intention of installing a bathroom. There is a 4 inch sewer drain for a toilet a few feet away.
Thanks

Pour a bucket of water down it, then look down with a flashlight if it reflects back it has a trap, if not and you smell sewer gas it's a clean out.

It shouldn't matter if the drain pipe has a trap or not because you should put a 2" trap right off your shower's drain before continuing the pipe to go tie in to your main drain. Don't forget to install a vent in the vicinity of your shower also and remember to give your shower drain pipe some nominal slope towards the main.