When radiator is out do the "Flip test"
Stand radiator up like it sits in the vehicle
Put hand over lower rad hose outlet
Fill up rad via upper rad hose outlet
Remove hose
Remove hand over lower rad hose outlet, water should drain out instantly, rad cap open and upper hose open lets in plenty of air
Now FLIP RAD OVER, upside down, NO WATER should come out
If any comes out it can have only come from blocked passages that couldn't drain
You can lay rad flat and fill it with HOT water and vinegar solution, let it sit for 15min, then flush
Repeat as you see fit
Do Flip Test again
Clogged passage will eventually rust out and leak because there is no coolant circulation so NO rust protection from coolant additives
It's why heater cores leak, block passage, rust starts, hole follows