Thanks everybody,

If you're in the US, the building code requires bathroom vents to run to the outside. However, bear in mind that in older houses, the vent ducting may have ruptured or pulled loose. If you're concerned, have a buddy shut himself in the bathroom, stuff a towel under the door and spray a shitload of air freshener or something with a strong distinct odor. Go around the house and see if you can smell it.
Good idea, thank you