What Nos said.
You want to be able to check the seal integrity without blowing them out (though, I suppose, then you'd know you had to replace them...) Which means you have to know the spec they're supposed to hold. Which will be in the shop manual, along with a description or part number of the gauge and plug used to perform the test, and the procedure for doing so.
As to saving a bit of money by doing it yourself: I could probably find you the steps and equipment required to, say, perform an appendectomy; I certainly wouldn't suggest you do it yourself. Some things are best left to others better equipped and more experienced and skilled.
But, to answer your question, you pressurize and read through the gear oil fill hole. Special diagnostic tool, as far as I know (I've never needed to have it done, but would take it to a shop if it was required.)