I started at my county extension office. They gave me the contact info for the state vet. lab. I talked to the vet at our state lab who said to bring the sickest bird I had. They did a live necropsy and used tissue from the trachea, which is what came back MG positive. It was $35 and I drove the bird to the lab 2 hours away myself.
I don't have any experience with non-MG IB.
Once they have MG, the whole flock has it and it will never go away. It will be passed to offspring thru the egg. MG is a weak-walled cell and doesn't survive long outside the body, but if you decide to cull, you will need to disenfect EVERYTHING- feeders, waters, shoes, etc. Basically everything that was in contact with things in contact with the chickens. It can spread from your shoes, to your porch, to your car, thru your house, onto other people's shoes, get transferred onto your furniture off of your clothes, etc. Scary stuff.
MG can lay dormant for a long time, or forever, never showing any symptoms at all. It can flare up in times of stress.
Only testing will confirm it in your flock.