First, see what you can surmise from the existing structure such as heating vents. With the grill off you can often see and poke around with a wire and figure stuff out. Electrical outlets are typically mounted on studs. With the plate off and power off, you can often learn where a stud is. Use that info and lay a tape out and given starting and ending points you can often have a good idea before you drill.

The nature of hand plastering often leaves a very slight wave pattern with crests on the studs. Try holding a flashlight against the wall in low light. Its a pretty sure bet on ceilings.