You can "sex them" easily by cleaning them. Egg sacs vs milt sacs are easily distinguished. Otherwise, it's kinda hard to tell, other than when they have their spawning colors displayed ... the males will have darker black pigment & it'll cover a greater area of their bodies.

That "mucky mud" stomach content is likely the fry or young of the other species in the pond, or perhaps even bugs/critters mixed in with bottom debris.