Thanks. I'll try to get to one before Christmas. I was researching this myself, and was surprised to discover that Garrison Keillor was raised in a Plymouth Brethren church in Anoka.
It's a denomination I have not visited, so it's interesting. I've rarely found any church that doesn't welcome visitors. It has happened a couple of times, but very rarely.
Still, I usually contact the church before attending, just to find out what the best time would be and to let them know that I'll be there as a visitor. It helps prevent any awkwardness that might occur (I speak from experience.)
So I've heard. He's gotten a lot of mileage out of it.
One of the elders in the local Brethren assembly will be happy to talk with you prior to your visit. One of his responsibilities is to greet visitors, explain the meeting format, and to anwer your questions. As a non believing visitor, you will be warmly welcomed. You will be asked to simply pass the bread and cup when they are distributed.