It's a very interesting question you ask about how people are diagnosed with having a hernia I will admit that I have no idea, other than my own situation - so I hope others will join this thread and enlighten us.
In my own case, I was discussing the swelling and the pain with the stoma nurse and she had no hestition in telling me I had a hernia (as if she was expecting it!) She offered to refer me back to the surgeon for a repair but warned that they were not very successful. I'm not sure whether she was referring to the operation or to the skill of the surgeon who might do it. Either way, I declined and said that I would live with it until it became unbearable.
She recommended hernia belts to stop it getting worse and I've modified their ideas to cater specifically for my own needs as their belts were practically useless.
I also have a hiatus hernia, which they diagnosed by putting a camera down my throat and having a proper look. This seemed like a much more practical and valid way of diagnosing something like this but it didn't help much because they did nothing further to aleviate the symptoms.
I hope this helps in your deliberating on what to do next