If the coolant is full when you shut if off then any pressure 'leaking' will occur in the next few minutes and with that magnitude of coolant loss you should be able to see the source though you may have to remove the plastic covering under the engine. Coolant will leave a white residue and is almost always visible especially on plastic parts such as on the expansion tank if the cap is involved. A bright white LED flashlight will highlight the residue. I would start with areas that are above the level of coolant when it is a quart low.

There are symptoms associated with a blow head gasket or other internal problems associated with coolant. Engine runs rough, loss of power, sweet smelling white smoke out the back, indication of coolant or oil in the coolant.

I am not suggesting a pressure test won't be beneficial. I am saying that I have never needed to resort to a pressure test to find any type of coolant leak in the last 5 decades. I love an excuse to buy new tools but as few times as I have had a coolant leak over the years I wouldn't suggest buying one. If you want go that route, I would take a trip to local radiator shop, or almost any chain type auto repair shop.

__________________
X5 4.6 2002 Black Sap, Black interior. 2013 X5M Melbourne Red, Bamboo interior
Dallas