Leaks are the bane of all bus repair techs. The problem is where the water enters can be a really long way away from where the water drips.
You didn't specify if you are working on a Type 'D' or a Type 'C' bus. The places to look are similar but not the same.
On Type 'D' buses the most common area of leaks are from around the window seal. You may also want to check around the area where the cross over lights were installed.
On Type 'C' buses the most common area of leaks up front occur where the wiper posts go through the body and the join between the bus body and the OEM supplied chassis.
Rarely when buses are old enough to be in the used market and sold for conversion most roof leaks have already been solved. But on Carpenter body buses they will leak sometimes along the inside edge of the rub rails that go lengthwise on the roof of the bus. They also sometimes leaked in around the front fresh air ventilator.