Similar topics have come up recently…
See Gerrie’s recent post here:
In short? There are some races where you will see only those racers in your category (and you can find them by performing the search Gerrie suggests on zwifthacks), but there is no way (yet) to toggle this on/off for all races.
When you do find yourself in the mix with riders from all categories and you aren’t sure if your Category peers are ahead or behind? There is only one correct option: go faster! If they’re all behind you? There’s no such thing as ‘winning by too large a margin’! And if they’re in front of you? Going faster is the only way to catch them. 
Ride on!