Flemmies are amazing but I can't guarantee, I just think she or he will grow a bit more. Onion smelling poos are marking territory, so I guess older than 3 months if male and older than 5 months if a female.
You can check if she has testicles, hold her on your lap as in pic#1 and with one hand check underneath without flipping over, that is because male rabbits are very smart and can retract testicles so you won't see them. When they are relaxed and not expecting examination you will probably find them easily.
Or you can do full check following instructions on this thread, there are clear photos and it's not a baby you should see clearly the pink part coming out or not when you apply some pressure
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Sexing rabbit pictures!
Here's another link to rabbit-savvy vets check Massachussetts there sure you will find one locally
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