I have an HP 50V P/S with current limiting, so I could do a simple test. OTOH, since you have to remove the TVS to test it, why not just replace it with a new one, they're cheap. Why waste the time testing it? Another point, they most often fail shorted. Here's one of many references, this one in a Microsemi "MicroNote" about TVS devices. In that case, it's easy to determine as you have a short circuit.
In the vast majority of cases, a TVS will fail in a shorted or severely degraded mode when overstressed.
This has often been the preferred failure mode for many applications, since the device would then
protect the remaining sensitive circuit from further severe transient threats due to its “electrically
shorted” shunt path to ground in front of the protected load until the device is replaced.
