A rental property of mine has a stairway with a light controlled by a switch at the top & a switch at the bottom. Either switch will turn the light on or off, independent of the other switch. A few weeks back, an assistant of mine removed one of the switches to do some drywall repair, unfortunately not noting how the wires were attached. Each switch was 3 wires, the one at the bottom has a red, a white & a black wire. On this switch, the red wire goes to the common terminal (this switch was not disturbed & the setup did work previously). The switch at the top has 2 black & one white wire. I tried connecting the top switch so that the white wire is common, this way I can turn on the light with the top switch regardless of the bottom switch's position, and I can turn the light on & off with the bottom switch if the light is left on at the top, but I cannot turn the light on with the bottom switch if the top on is off. So I'm wondering how I can determine the connections for the top switch. There must be a way to do this with a volt meter. Suggestions?
TIA
Dan