If the spur is within the pit from where it grows from good chance it is female and if it falls outside of the pit its most likely a male. Honestly, I think the best way to tell is how the tail tapers(other than probing). I had about 30 Argentines last year that I just went through, eye balled them by tail taper, and went back through later with a probe and I only missed one.
Later-
Matt Schubarth
Pet Nebula

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