In almost any internet debate there invariably comes a moment when someone will accuse another of using an ad hominem attack or argument. Unfortunately on the internet, any disagreement or attempt to disprove someone's argument is perceived as an attack on the person. But that fundamentally misunderstands what an ad hominem argument is. The key to identifying an ad hominem argument is to consider whether or not the response is responding to the original argument, or if it is ignoring the argument and responding directly to the source of the argument.
While it is common for people to mistakenly interpret an argument against their position as an attack on themselves, what is almost universally misunderstood is that not all ad hominem arguments are fallacious.
To put it simply, all ad hominem fallacies are ad hominem arguments, but not all ad hominem arguments are ad hominem fallacies.
What many people don't realize is that an ad hominem argument is a perfectly valid way of arguing. For example, say there was a video on the internet of someone dressed in University of Utah clothing giving a negative critique of BYU's football team. We could easily dismiss a negative argument about BYU football from the Ute fan based solely on the fact that they are a Ute fan, and not unbiased in their assessment of BYU football. Saying, "Well they are a Ute fan, so of course they will say that." is an ad hominem argument. It does not respond to what was said, but only attacks the person making the argument in order to undermine their argument. This can be a perfectly valid argument.
Now if the person in U of U clothing turned out to be the U of U football coach, then dismissing them as a Ute fan would turn the ad hominem argument into a fallacy. A football coach probably has a sound basis for his argument, and refusing to address his arguments would make it a fallacy.
Sound ad hominem arguments are used more frequently than people realize. Statements such as, "They are paid to say that." or, "They are an advocate for [blank] so of course they would say that." Looking at the source of an argument and dismissing it based solely on the source is an ad hominem argument. Whether or not it is a fallacy is another matter.
So despite what many people on the internet immediately assume, to be dismissive of someone because they lack the necessary expertise can be a valid argument, and is not automatically a fallacy. But if the argument is not considered at all you better have a good reason for dismissing the source and not considering the argument. If not you run the risk of committing an ad hominem fallacy in your argument.