Rust! Rust! Rust!
 

I own a 1995 sonoma (from Wisconsin). It has 93000 on the odometer. The body panels are really nice. The frame, however...
There is a lot of rust under there. I had the bed off it, the fuel tank is extremely rusty.
When I bought it from my father, I took it on a trip across the country. It had no problems along the way, got me to college.
What tests can I do to determine structural rust?

Also, if it is structural, what do you do about it? In other words, I know the history of this vehicle. Do I look for another vehicle that may have severe problems in other areas, or do I stick with this one, strip the rust off of the frame, and use a coating chemical?