what they'll do is put a cooling system pressure test kit on, and pump your cooling system to a 17 psi or so. if it holds at 17psi for a couple of minutes you're good to go.

if theres a leak lets say at the head gasket, it wont hold the pressure very long, but you wont see coolant spilling on the ground.

buuut if its anywhere on your radiator or hoses, you'll see the fluid spewing right away.