If you suspect the radiator is clogged, Can you see the tubes through the
radiator cap hole? Will a small mirror get down into the opening?
Sometimes solid pieces of crud sit on top of the tubes.

You can take plastic mirror material, and make a skinny mirror, and
shine a light against it to see the tubes.

You can also remove the radiator, hold it horizontal, fill it with water,
and slosh it sideways, to shake/slosh/wash out the solid pieces of crud.
This also will show you if water flows through the tubes.-Bob