I used to work in a radiator shop. If they still use procedures like I did in the early 90's here's what they will likely do. If the leak is big enough, they will have to repair it before they can test the radiator. They'll pressure up the radiator with air and dip it in a tank. If the radiator has a lot of tiny bubbles coming from the tubes in the central portion of the core, it is likely bad and will need to be replaced. Air can escape holes smaller than water can, so some of these types of leaks may not yet be leaking water. If someone couldn't afford a new core, we used to add sodium silicate and seal up the tiny leaks, but that does not last long. Radiators that are truly shot you can press your thumb nail against the tubes and easily break through. They wear out from the inside out.