Does anybody, especially those who experience Flyaway / GPS or Signal Problem, monitor UAV Weather Forecast ( http://www.uavforecast.com ) to indicate high KP and said "no" good to fly at the time, when you flyaway / GPS problem / Signal Problem and so on ?

and how to check past condition of UAV weather ?

To answer your first question I check K INDEX on the day of every flight and do not fly if readings are above 3/4 and then with caution.Cannot comment further as I have never experienced flyaway !touch wood!

Has anyone given thought to why so many are giving so much credence to the K-index thing, yet consumer GPS receivers and phones aren't giving any problem?  I use both for hours every day.

NOAA Space Weather Site

K index NOAA page

Some info in the above links. Space weather can effect everything from power transmissions to GPS. Mostly in high latitudes. But use the above sites for in depth reading.

Personally I have experienced problems with GPS and earlier with Loran. It is rare, but it happens.

I find that it's better to check the index before I fly rather than after...

Found a nice iOS app "Solar Monitor" for that. Ig gives an alarm when kp index is high.
That way I don't have to check myself much, Jean I am a lazy guy.