the eggs were all infertile as I expected, after this they again laid eggs 4 times, but they were all infertile as well and I came to the conclusion that there are no males in the tank. planning on buying a few more cause there is no way to sex them.
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.