My garage was added onto the house around 30 or 40 years ago. I want to replace two of the light switches. There are a total of 3 switches controlling this light (the third switch is previously replaced).

I believe that makes this a 4-way switch circuit. The first switch looks like a typical 3-way switch, so I'm guessing it's at the end of the run. It has two black wires and one white. I do NOT want to depend on their color codes though. How would I use a meter to find out which wire goes where on the new 3-way switch?