one way would be to drop the pan, then apply power and ground to the connection on the case, then listen and feel for the solenoid[s] to activate.
that would be my first step.
if they both work at the transmission, i would then work backward in the harness to see where the connection breaks.
if the solenoid[s] does not work at the transmission case connection, i would then go directly to that solenoid with power and ground.
if it then works, the wiring to the case connector needs repaired.
if the solenoid doesn't work, it needs replaced.
just my way of doing things.
your mileage will vary.