when sexing rodents the main thing to look for is males that the males penis and anus will be further apart than the females.
this site may also help you

Sexing Gerbils | eGerbil

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animals are my life

i have:
2x dogs (rottweiler and a jug),
1x african grey parrot,
2x rosella parekeets
4x syrian hamsters (baby, mocha (wiggy), peaches and daisey),
2x winter white dwarfs (sunshine and harry),
1x female leaf insects (doesn't have a names),
1x female corn snake (awaiting eggs),