What is the best, proper approach to measure current flow at bonding points? How much flow is too much?
With corrosion, voltage differential between bonding points is measure in millivolts. With such small differences, have a large effect, it’s practically impossible to properly measure milliamps with commonly encountered meters. The proper approach is to measure the millivolt potential at each bonding point in reference to an anode.
Depending on the make of your vessel, no matter if it is fiberglass, aluminium, or wood, each hull material will have its own proper millivolt range where the underwater metals are properly protected.
A write-up on how to test your own bonding is outside the scope of an AskPYS. A good starting point would be Nigel Calder's Boat Owners Mechanical and Electrical book. He has a very multi-page test procedure.