Interior Top Plates (Runners and Main)
To get a sense of where the top plates are located in the attic, walk through the rooms below and make note of where the walls line up with the ceiling, pay attention to distance between the walls and known locations in the attic like bath fans, ductwork, gable ends, etc.
Plumbing pipes can also lead you to the top plates. From there you can follow along in either direction until they terminate or intersect with another top plate.
When sealing top plates make sure to seal any holes or wire penetrations
Plaster ceilings can make it especially difficult to find top plates. Measuring the distance from the walls to the hatch or gable end can be helpful. Once you are close to finding the area, look for the strip of wood without plaster on either side. Air seal with expanding foam.