We've used them here with the same walls. Stud finders work by sensing the difference in the density of the areas it is moved over. When it's over a stud, the area is denser than when it isn't, and the difference shows up on the readout. Whether your wall board is plaster or sheetrock shouldn't matter. They are a little more sluggish to give you a readout, so you need to move the finder more slowly over the plaster wall while searching than you would with sheetrock.
A stud finder may or may not work depending on the thickness of the plaster.I usually back up my findings with a nail. If you want to prove to the Ms. how studly you are just run the stud finder across the fly of your pants. ( It may help to have a zipper).
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