Testing parallel IGBT module
There are tests you can perform to gauge the "health" of a module. Essentially you need to characterise a number of healthy modules. The minimum number of failed devices essentially comes down to device tolerances and measurement accuracy.

What you need to consider is what types of failures are you looking for? 


shorted gate-emitter. can be detected via G-E impedance
shorted collector-emitter. can be detected via C-E impedance
open gate-emitter (bondwire, damage...). Can be detected by a change in characteristics
etc... 


There are four characteristics worth determining for the complete module


\$C_{iss}\$
\$C_{rss}\$
\$C_{oss}\$
fwd voltage drop.


An IGBT die datasheet will usually provide the small signal \$C_{iss}\$,  \$C_{rss}\$, \$C_{oss}\$. You cannot just take these values and multiply them by 200 as there will be additional stray capacitance within the module under test.

Once these values are measured &amp; captured for a number of healthy modules, you will be able to determine a spread. The number of failed devices you could possibly detect then comes down to  the deviation. The loss of one device may still fall within the possible healthy range. 2? 3? once you have some empirical evidence you can come up with a detectable number.

The final method is to mount the module on a heatsink and heat the heatsink to a given temperature (50C, 75C). A significant temperature above ambient to mitigate change in ambient when testing on different days.

Gate ON the IGBT stacks (9V or the correct 15V) but ensure you measure the gate voltage with reasonable accuracy (not a 9V battery when it is really supplying 8V...). Then with a high current source with a resistive shunt to measure the forward current accurately, apply this source Collector --> Emitter.

Measure the Collector->Emitter voltage.  The actual current to be applied should be say... 50% of the maximum device current.  With the loss of one or many devices this \$V_{ce}\$ will rise.