How did that authoritative source get their knowledge? If it wasn’t through the scientific method, then it’s not knowledge. So you can proxy your knowledge through someone else you trust that did the legwork of going through the scientific process, but that’s not another way of obtaining knowledge, it’s just the scientific process with extra steps.

Personal experience can also result in knowledge, through the use of the scientific method. You can drop an apple and see that it falls. That’s the observation part of the scientific method. You can go further and try to figure out why that happens, by using the scientific method yourself based on your personal observations of the apple.

There really isn’t another way. It’s the scientific method all the way down.