hetfield: If I read your post correctly, your question is "how does a pilot tell whether or not is gust direction significantly different from the prevailing wind?"
The answer could be hidden somwhere deep in my MET notes or Annex3, both of which are outside reach now. I suppose it would be coded as 27015kt 190V330G28 then. If it is plain simple 24015G27 the criteria to include the variating direction code had not been met, i.e. the measured gusts are "reasonably" close to the prevailing direction. What do you think?
Sincerely,
FD (the un-real)
PS: One station on our network regularly reports something like V5G18mps; funny how this causes havoc during preflight to stay legal for dispatch, especially on contaminated runways.