How To Estimate height?

On this forum I always see you all able to estimate a mans height, but how?
I am always so confused and can never seem to have any idea..
Any tips? For example, I'm a 5'5 guy, how tall would a man have to be where the top of my head would just reach the edge of his shoulder.. how tall would someone 6"8 on to me?
I always see men in public but can never seem to guess and can't will the nerve to ask.
What are your tips.. and what thresholds for your height do you use? (For example how tall is someone when you just reach their chin.. or just reach their chest..)
Thanks!

http://www.mrinitialman.com/OddsEnds/Sizes/sizes.html

try this

or try this...
http://www.giantdimensions.com/index.php?page=heightcomparisonviewer

If you see somebody exceptionally tall, observe their headtops relative to know heights. For example, in the US, a residential doorway is 6'8", a commercial doorway is typically 7'0". Walking past a tall guy, I observe where my direct eye level falls on their body if my head is level. For example, at 5'9", my eye level is right at the nipples of a typical 7'0" man, or a the shirt collar level of a guy who's 6'5" (shoes being similar). And you can extrapolate above and below that.