Been there done that.....
Luckily with garters the difference is apparent within a short time..
The ladies get huge compared to the boys.
When I found my eastern milk I thought for sure it was a he, then she dropped a nine egg clutch.
My first ever snake delivered 28 live babies....
Here is how I sex my snakes, I take the males out of the deli cup labeled "Male" and I take my females out of the deli cup labeled "female"
that works most of the time