there have been plenty of posts on this. type probe or pop in the search bar at the top right of your screen at he main page and it will bring up posts mentioning this. in a nut shell..... just look.
Well I for one will always probe to verify the sex. I've been popping since day one but have recently been frustrated about what I thought was a female. Probe it for confirmation in my opinion. Reptilebasics.com has a nice probe set for 19.99 along with a carrying case ( which I ordered promptly the other day ). Hope that helps ;-)
I pop. For me, it's more accurate than probing. And sticking a foreign object into my snake doesn't fancy my interest.
If done incorrectly, you can push the probe straight through the genitals....
As for learning how to sex, I would find a local breeder to show you how. Seeing it in person is much better than watching videos on youtube. And by having someone mentor you, you can know exactly how much pressure you need to use.