A leak test only shows integrity. It may have been badly repaired in the past, with tubes crimped shut, full of debris, etc.
I think there's a flow rate test (how much volume passes in X seconds), but even that doesn't guarantee the surface condition of the interior - where the heat transfers from the liquid to the tube wall.
Sometimes the choice comes down to: it is cheaper to do it right (new rad), or do it twice (repair, install, fail, repeat)?
