Another trick we used when installing recess lights (Hi Hat Cans we called them.) You never know what is behind the ceiling if you are retro fitting on the first floor. So we would take a piece of #10 wire and bend into the shape of a "C" the top leg would be 1/2 the diameter of the can. For example a 8" can, we would bend the top piece of wire 4". Then we would use the height of the can say 6-8" and make that the middle part of the "C" on the wire. The bottom part of the "C" was what ever was left over.
We would make a small hole in the center of the can. Insert the 4" top part of the "C" into the whole and start spinning the "C" around. Now if you can make a 360 deg spin all the way around, then slowly raise and lower the "C" all the way up the 8", you would know that the cavity behind the ceiling was clear.
If you hit a floor joist, pipe, wire, etc. the spinning "C" would stop or make a metal sound (Ting)when you hit something like a pipe. We would check all the holes before cutting. This way if you hit something in one spot, you might have to relocate a whole string of cans to keep them in a row.
