I've always like the low pressure warning light. It's reassuring to watch it go off after the engine warms up a bit. I use to know the Napa PN for the sender, but lost it. It would trigger the light when the pressure was under 2-4 lb.
I bought a Stant cooling system pressure tester years ago and use it often. It is a great way to find leaks and let you know if the system will hold pressure for a long time. Much better to find a problem in the garage before a race meeting than on track.