Can your unspayed female cat tell that your male cat is neutered?

Without getting near the cat you may be able to tell if it is fixed if it has a clipped (cut shorter) ear(s). This is done for feral cats so that they can assist in population control and so they do not have to go through being recaptures.

If you are up close to the cat you can check the cat. With males it is often easier for most because it is either there or its gone (their private parts that should have been the back-most thing on them at the base of their butt and tail). With female cats unless they were fixed recently it is near impossible to tell. You can attempt to look for a scar on their belly.

Lots of low cost spay/neuter clinics are starting to make it easier to tell if a cat or dog is fixed by putting a tattoo on their belly (both male and female). The tattoo is normally just a colored line. If they have the tattoo they are definitely fixed.