I use a home-made 12V battery charger to zap nicad cells. Short-term, it will deliver 20A, so the zapping must only be for a second or two. H&S-wise, throughout the following procedures there's a risk of explosion, so take whatever precautions you feel necessary.
Immediately after the quick zap, I measure the cell's o/c voltage. If the cell has not completely gone, this is typically around 1.2V. However, this does not necessarily mean that the whisker has been blown away.
So next I look at the s/c current (meter on at least a 10A range). There will probably be a quick burst of current, rapidly dropping to very little.
I then go back to measuring the o/c voltage. If the whisker has gone, this might be (say) 0.5V, but it should slowly be creeping up as the cell recovers from the s/c test. But if the voltage is falling, the whisker is still present.
If the zap appears to have worked, I immediately charge the cell. When charged, I usually do another s/c test. The current will start high (typically an amp or two for AAA, 6A for AA - and even more for C and D sizes, so take care not to damage the meter).
With a good cell, the s/c current will fall fairly slowly - so don't keep it on s/c for long, as the cell will get hot and might explode. If the current almost immediately falls to near zero, it's probable that the cell has little capacity left, and it's a gonner. If it looks like it might be a goodun, I again immediately measure the o/c voltage. Like before, it should be rising as the call recovers from the s/c.
Finally, a give the battery a normal charge - and after that it's a case of 'see how it goes'. I've had some rejuvenated nicads last a further year or two, but some have been pretty useless. [Note that although NiMH cells are different from nicads, I've also had some that have responded well to similar rough treatment.]