Partial Discharge Analysis, both off-line testing and especially on-line monitoring, is a good (fast, easy) way of testing the condition of generator stator insulation while the nachine is in normal operation and under all the usual thermal, mechanical and electrical stresses that it sees in normal use. Besides Doble, other leading PD instrument companies are IRIS Power, PD Tech, Power Diagnostix and Cutler-Hammer. They all do a pretty good job of measuring PD and cost roughly the same (e.g. 50K for one portable instrument + 15K installation per machine). Whoever is closest/easiest to deal with would likely be your best bet if you want to go the PD route.
Good luck and all the best to you.