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I do almost always use a way simpler, faster method:

1. Remove the abdomen or cut off the last segments - check the relevant literature beforehand not to damage important structures! Sometimes it is essential to remove the whole abdomen.

2. Soak the abdomen in warm water with detergent for some time (a few minutes is enough for small flies, 1/2 hour or more for large flies)

3. Dissect and examine in water (or glycerine, but then you have to wash before the next step or permanently store the genitalia in glycerine).

4. Drain the examined genitalia and glue them on a slip of cardboard, which can be pinned under then original specimen. Once the surface pollinosity of the abdomen is properly visible, the abdomen is dry enough for mounting.

Dissecting is slightly more difficult as muscles have not been obliterated by KOH, but on the whole, this system is way faster than doing a 'proper' genitalia preparation and yields perfectly usable recults. And as a bonus, the cardboard-mounted genitalia and very easy to examine and don't get separated from the specimen.

Using KOH is overkill for mere identification purposes.