I have a threaded adapter that converts the bleed screw thread (M10) to a standard air fitting (1/4 NPT) - male M10 to female 1/4 NPT .

I then put 17-19 psi (less than what the cap is rated for) into the cooling system and look for leaks. I would probably turn the key to run and turn heater up to 90F to open thermostat. Then look for leaks.

If I don't see it after this, I will add one half bottle of Coolant Ultra Violet dye and use UV sunglasses and a UV flashlight (all bought on Amazon for cheap) to find my leak...The last hard-to-find leak was coming from the Heater control valve...which I had previously rebuilt with new seals....Wooops!