I can imagine finding the thickness of the dry wall in either of the two ways described: 1. drill through the dry wall in a place which will be covered and where there is no stud. Drill through very slowly so you will feel when you break through the plaster.. Stop the drill, wait for it to cool off, pinch the drill bit with fingers just at the edge of the wall, and remove the bit. Measure the bit that protrudes beyond your pinched fingers.
Other way, I'd do both, is drill through the wall where there seems to be a studd, stop when the resistance seems to increase as the stud is probably harder than the dry wall, and measure as above. Hope that's not too much detail? I also agree, don't put up a cabinet with nails, use long screws into studs. Tom
