I own 3 repeaters (six meters, two meters, and 440) and I maintain three more for our club. They are all GE Mastr II's, so that simplifies matters a bit. The duplexers are a hodge podge of different brands. Cel Wave. Wacom, Tx Rx, Sinclair, Motorola etc.
I use a Motorola 1200 service monitor which is around 30 years old and lacks a lot of features that some of the newer ones have. I also have a HP 141T spectrum analyzer and tracking generator, which is a necessity when maintaining repeater transmitters.
While not the best way to tune duplexers, I use the Motorola 1200, a couple of 6dB "pads", and a Sinadder. On the receive pass adjustment I tune for maximum sinad on an ICOM R7000 receiver. I do the same on the transmit pass adjustment followed up by slight adjusting for maximum RF power output with low power. On the rejects I tune for minimum sinad on the R7000. This has always worked for me and none of the repeaters has any de-sense. I can "look" at the pass - reject on the spectrum analyzer.
Of course if you have access to a good network analyzer, that is the way to go to tune duplexers. I just don't have $50,000 to buy one.
Dick AD4U