Hey, quit squeezing on that poor girl's tail.Seriously, if your popping technique was good enough to determine the two males' sex, then you ought to trust that you've done it correctly on the presumed female, too.
If you want to learn how to probe snakes, ball pythons are a lot easier to practice on then, say, hatchling corn snakes. The thing I always wonder about if someone whom I don't know has done the probing, is were they too cautious when they got to what they thought was the end. If you use plenty of lube and twirl the probe while you're inserting it instead of just trying to push it straight in, then it should slip w a a a y down (9+ subcaudals) for male ball pythons. For females, 4 to 5 subcaudals is really quite common.