The drain cock on my cooling system is connected to both the bottom of the radiator and to the block. When it's opened, it drains both. I've found that compressed air also helps get the last bits of coolant out of the system. I stretch a heavy piece of inner tube rubber over the radiator cap opening, poke a hole in the center and stick the nozzle from my air blow gun through the hole to make a poor man's seal. With the pressure set to 5-10psi I use the air to purge what's left in the system out the drain cock. That gets an extra bit to come out. The air can't blow into the top hose because the thermostat is closed, but it does pressurize the engine passageways through the lower hose. I fill the radiator with distilled water and flush it but don't use chemicals. I fill with coolant and leave the drain cock open until I see what comes out change from water to coolant.
If you don't have a drain cock, I guess the best you can do is remove the lower radiator hose and let that drain both the radiator and the block. The hose is at a low point on the engine, it should allow most of the coolant to drain.
I test my coolant every year with test strips, you can get them at NAPA or online. They measure how much additive is left in the coolant. The manual calls for 2 year changes, I've found mine goes 4-5 years before I see the beginning of a decline in additive, so I change it every 4-5 years. You can also send the coolant into a place like Blackstone labs and they'll tell you exactly what state the coolant is in.
Hydrometers are useless for measuring temperature protection, you need a refractometer. I took an auto repair class at the local community college, they had a dozen hydrometers to demonstrate that none of them worked, they weren't closer than 10 degrees.