Pulling an outlet or switch, peering in is a good trick! Stupid simple...why didn't I ever think of that?
Generally, unless you really want to preserve the old plaster my best stud finder is either a sledgehammer or 23oz framing...
Destroy & rebuild!
Never know what your going to find behind those walls...and indeed most were well off 16 center. 20 or 22 isn't uncommon, as is truly dimensional lumber which is gonna throw everything you know off whack.
Best thing ever found behind a wall ?
Old buddy who used to flip houses, bought one circa 1920... Run down & down more. Multiple dumpsters, gutted to studs in Armonk ny.
Swinging away and hit something solid. Way solid. Clang!!!
Metal on metal...
Got a few smaller bars out to peel away & uncovered a cast iron stove/fireplace that had been walled off in expansion.
They completely changed gears, swapped the entire floor plan, restored & made it a focal point of the house, more like the original layout we figured.
I've uncovered dumb waiter shafts, unknown electric & all kinds of stuff...ya just never know.