Walls and Ceilings - load bearing walls

I just purchased a home with a flat roof. I would like to remodel both bathrooms. The house was built in '58 and the walls are plaster with wood beams throughout and the exterior of home is concrete. I've been reading online about how to identify load bearing walls. I can't find anything regarding flat roofs though. I would think the roof rests entirely on the outside walls of the house but I'm no expert! I'm basically wanting to knock down the walls inside the bathrooms but not the walls outlining the bathrooms. Every wall in this house is so solid you would think each one was a load bearing wall!!