Drain boiler to repair some leaking pipes & pressure testing

I have a new simple one zone residential boiler. (newly installed). Can I just turn off the power switch at the boiler, hook up a hose to the drain and open it up into the floor drain? This is what I always did with the old 1950s Janitrol boiler, but I just want to make sure that's the correct procedure.

The plan is to replace a few nipples at radiator valves that have rusted out. and also add some packing material (teflon or graphite) to all the valve nuts to reduce the dripping that probably is the cause of the rusted nipples. Once I do this, I'm wondering is there is a professional way to pressure test the system to see if it will leak anywhere? It seems like there must be a better way to test for leaks without refilling the whole system and looking for leaks. Most of the pipes and valves are 1912 so obviously there may be other leaks than what I can see.