There are tools that use a vacuum pump to test for leaks at low pressure values but all it will do is tell you it has a leak. It won't help you spot where its coming from.
Most of the time a person can see where a leak is coming from while the part is still in service but it might only be the general area. My own experience is that once they leak, it's usually time for a new core anyway. There is usually a lot more than one thin spot inside a leaky core and the radiator would look like Swiss cheese to try and repair the leaky tubes. If I was to take the trouble to remove the tanks, I would be installing a new core if one the right size is still available.