I use this kit regularly and works well. You can also rent from parts store with money up front when returned money given back.

Powerbuilt 22 Piece Cooling System Pressure Tool Kit, Service and Leak Detection for Cars and Trucks, Coolant Adapters, Storage Case 940427

 

Depending on just what you're trying to do...

I diagnosed my head gasket leak with a running coolant pressure test.

The rig was simple - a tee in one of small hoses running to the degas bottle, connected to a pressure gauge with enough hose so I could watch it while driving.

 

I had a coolant leak and needed to be able to pressure test above 20psi. Ended up being a cracked head above the long injector cup they put in 5. I bought a cheap Vevor kit on Amazon, it worked but caps are pretty crappy.

 

A running coolant pressure test probably would have diagnosed this.

Perhaps better, because it is done on an engine at operating temperature.