When it comes to testimonial knowledge, the obvious difference between it (and say knowledge from experience) is that testimonial knowledge is not direct. It is in fact indirect. Since this is the case, some internet skeptics like to give lip-service to disregarding testimonial evidence.
But why should we give testimony up? When you think about it, we live almost everyday based off of testimony. When I get sick, I go to the doctor and get his opinion. When I’m a toddler, I lean on the understanding off my parents. When I want to know about rings, I see a jeweler. I could go on and on.
Now right off the bat someone might object, “But just because a doctor says X, doesn’t mean X is true! Appeal to authority fallacy” Right, we already knew that. And no, it’s not a fallacy. The objection in other words is misplaced. We should believe the doctor over others because the doctor has the knowledge from experience that others don’t. He’s a professional who has a priori and a posteriori medical knowledge that the average joe just does NOT have. We are not in fact saying that just because a doctor says something, it must be true. But you better believe that I’m going to believe the doc over you when it comes to medicine.
On the other end of the spectrum, we should be skeptical of someone like my lying cousin. To which he might reply, “But just because I have lied, it doesn’t mean I’m lying now! Ad hominem!”. Once again, we already knew that. And it’s not an ad hominem. We aren’t saying that what he says is necessarily false. Rather, we should suspend exercise caution when he claims something.
So the basic principle is this:
Believe in testimony X, unless you have a good reason to think the person is mistaken, lying, etc. If you have no reason to doubt what someone is saying, then why not just go ahead and accept what seems true?