You can install it in the lid, hook up the coolant lines, but leave the lid off while you cycle the key on. When the pump is running, you can then verify if you see any leaks before installing the lid.
There's gotta be a way to test it under low pressure on the bench. Maybe plug one end with a expandable plug and plug the other end with a tapered plug inserted tightly, drill a small hole in it then insert a ball pump needle it it hooked to a small hand pump with a gauge. I would think to pressurize it to just under what the cap is rated for and submerge in a bucket/tote of water.
Just thinking outside the box here.
Grab a couple balloons and fill them up then tie them to the ports inflated and let it sit to see if they deflate quickly or not. The coolant has pressure inside—> out but also the manifold boost asserts pressure from the outside—> in. A little different than a traditional radiator.