It's good to know the gender
I just spoke to a caregiver who sent in a cat for sterilisation. It apparently had some wounds that needed to be stitched up, so the cost came to more than she expected. It became more complicated, because she mentioned that the cat was a pregnant female, and the vet clinic told me it was a male cat. She said that she thought it was a female cat because as she was feeding it, it got fatter and fatter.
It's a good idea to check the gender of the cat. Firstly, the female cats are more difficult to do, so if you send in four females, instead of four males, the vet is not going to be very pleased, as their schedule will be out of sync because females take so much longer. Secondly, it's just a good idea to know!
When you are feeding the cats for example, you can surreptitiously walk around behind and take a quick peek.