Personally:
1) I was having to "burp" the cooling system once every other week and progressively got to where I was burping it once a week or so. When I say burp, I would remove the thermostat housing, fill from the radiator cap until water filled the block and ran out the thermostate opening. I would reinstall the thermostat then start the car until running temp and the thermostat opened. All the air would come to the radiator so I would top off with water and carry on for another week or so. LOL Also helps if the car is on ramps to raise the radiator filler neck higher then the rest of the car.
2) I could tell a loss of power. I am picky about my car and notice small things. I could tell that there was a power loss from a cyl or two buring water as well as gas.
3) Could smell coolant burning while I ran coolant. Mine became eventually all water and I would experience a little sluggish startups with a small amount of white smoke.
