Air in the Cooling System

I have been reading a lot about burping and bleeding of air from the cooling system. I am curious, how can you be sure there is air in the system? How do you tell how much? How do you bleed it (properly)? and How do you know you got it all out?

Have checked the archives and other posts, but the answers only make me have more questions. Can someone please explain proper proceedure and how to tell if it needs to be done. Have a feeling that others may be asking same question.

Thanks,

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William
76 240D - 550K miles
78 300D - 200K+ miles