A quick check if your household water has about 60 lbs pressure. Put a hose on the one fitting. Since you probably have an old oil cooler hose use it and close it off either by folding over the hose and using a clamp. Or installing a plug and clamp after cutting the hose. . The use of the old oil cooler lines allows you to quickly change from one oil cooler test to the next one.

Take another old oil cooler hose and cut it. Install a fitting to hook up to a garden hose in the cut end. This will allow a test up to your available water pressure. It is better of course to take it higher to say a hundred pound test but probably good enough to spot a bad oil cooler. If the cooler drips it's an ebay item.