It involves doing what Jeff said and does require a sensitive touch. Once you know what you're looking for, you'll never need to probe again.
Begin by running your thumb and fore inger down the animal's spine and belly, respectively.
Apply firm, consistent (but not crushing) pressure as you do so.
Once you pass the vent pay attention. You're feeling for the hemipenes. You will feel a small bump with your forefinger as you pass the hemipenes. This bump will come anywhere in between the 5th and 10th ventral scale. If you feel the bump, you can be 100% certain you have a male. False diagnosis of females can happen, especially in neonates. I sex baby BRBs the day they're born using this method. On babies, I will use my ring finger or pinky instead of my forefinger because I have a little more sensitivity here.
If you can, practice on a large male. There's no mistaking the hemipenes.
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Mike Lockwood
www.tooscaley.com