What is the most reliable indication of a faulty variable displacement hydraulic pump?

Variable displacement will result in variable current i.e. load. Hence current will be UNIQUE. This can be one indication. If it is stuck at low or high displacment, then variation in volume will be less or negligible.

according ot the type of pump such piston or gear  so the most indication  we can measure it depended on :

the amount of slip  which control the pressure of pump

operation cycle of pump