Find the attic pull-down stairs in your home. The stairs are usually on the ceiling in a hallway. The access is rectangular and has either a pull rope or a metal ring to pull the stairs down.
Climb the stairs, taking a flashlight with you.
Stay on boards between joists when walking in the attic. Look for the main plumbing vent that goes through the roof of your home, usually near the bathrooms.
Move insulation as necessary to find the plumbing pipes that run along joists in the attic. Some pipes are clearly visible along walls or connecting to the truss above.