Fairly straightforward to determine males. Just net the fish out, gentle pressure and strokes towards vent, milt will extrude on sexual mature males. Their testes are large and simple (not high up / compressed near air bladder or corkscrew structure like synodontis) so easy to express milt. You don't need to knock the fish out for this but you can if your nervous.
Just having males present doesn't mean they will naturally spawn even when females are dropping eggs. It's easy to dry fertilise eggs & milt in a bowl.