I agree that the majority of hognose hatchlings and neonates can be sexed visually going just be tail length alone, this is my preferred method.
As far as body shape, this doesn't seem to really come into play until 6-12 months, depending on the growth of the indvidual. Females obviously become more rotund than the sleeker males and tail lengths at this size should be more definitive, but there are those darn anomalies that still defy the standards such as large males and undersized females both with non-determinate tail lengths, this is where probing is invaluable.
In regards to my own western hognose collection as well as the animals I've hatched out over the last 2 years, I've been able to correctly identify the sexes with 90% accuracy. The remaining 10% have been individuals that fall into the non-determinate tail length type. Considering that those specific animals are ones that I am not selling, time will determine their sex as they mature. Were these animals that I were going to sell, I would either have to sell them as non-sexed or would have to probe them prior to listing them for those that want a specific sexed animal.
To be honest, I think probing a hatchling/neonate is a very risky endeavor and the risk of internally damaging the individual is significant. I've heard of cases where females were accidentally punctured therefore giving the prober the false result of being a male because the probe went to deep. I've also heard of males being equally damaged due to probing too deep. Males can also give false-positives when the probe doesn't go into the hemipenes correctly, perhaps the probe is too big or it's just not going into the hemipenes and hitting the "pocket" instead, wherein the male is incorrectly identified as a female.
For western hognose, immature sexing should be attempted visually first and probing or popping as a last alternative. When it comes to choosing between probing or popping though, I choose to probe as I've never acquired the "feel" for popping and considering that I'm missing part of my left thumb (due to an accident), it's kind of hard for me to handle a snake in that way anyways.
But I agree Gregg, when mature, Western Hognose are in fact sexually dimorphic. If you can't tell an adult female from an adult male, something is very wrong! 
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Troy Rexroth
Rextiles
