If you take out the dome light housing you can see the double wall construction in that area, and you can run a flexible piece of metal or a wire up and over into the cab roof. I don't know if that would help to get a better estimate of the spacing between the two layers of the roof.
You could also use a hole saw without a center bit, and after removing the old headliner you could make a hole in the sheet metal on the inside to see how much space there is. You might also take a look at instructions for some of the custom headliners that have lights and radio/sound options, since they may require you to make extra holes for wiring.
Bruce