Typically to test a radiator (and the rest of the cooling system) you put a cooling system pressure tester (basically it's a hand pump with the proper adapter to attach to the neck where the pressure caps fits and a pressure gauge), you can buy or rent them. You pressurize the cooling system to 15 psi and look for leaks. A radiator shop will place the whole radiator into a tank of water and with the proper plugs and adapters pressurize the radiator to 15 psi with air to test and find leaks. If you have the original rad with the plastic tanks and aluminum core don't even waste time testing it. You can't properly test a rad with a shop vac. Good luck finding a rad shop. It is getting harder and harder to find one, the newer rads are not repairable and most of them have gone out of business. There are still a few staying alive because big rigs have repairable/rebuildable radiators. If you have ANY doubts about the radiator the best advice is to just replace it. If you blow the radiator you *could* overheat the motor and the cost of the rad is going to look cheap compared to what you have to do if you cook the motor!
David Teitelbaum