When you said "bubbling in the radiator", I assume you are looking for a possible blown head gasket and looking for combustion gases entering the cooling system.
Usually, the best way to check for this, is when the engine is totally warmed up(t-stats open partially or fully open), and under a hard pull= like going up a big hill, or running on a chassis dyno.

Now, if I was a farmer and wanted to troubleshoot for combustion gas possibly getting into the cooling system, then I would purchase about 15 feet of "radiator overflow tubing" and route that tubing from the overflow tube, at the radiator neck, over to the driver's side mirror area. {NOTE: Must have a KNOWN good radiator cap or new radiator cap}
Then you use some wire to hang a glass jar or clear, plastic bottle from your driver's side mirror bracket, fill your jar/bottle with water, stick the long, overflow hose into it(after engine is already warm!!!) and go drive up a big hill, pulling a load.

Now, as engine temp slightly increases(pulling that hill), you may see a few "expansion" bubbles(normal), BUT if the engine is truly getting combustion gases entering into the cooling system, then you should see lots and lots of bubbles. Really bad cases actually blow most of the water out of the jar/bottle, and really, really bad cases the truck is probably splitting coolant out the OEM radiator overflow hose already, once he even pulls off the road or evens stops for coffee.(Those cases don't even need testing).