I believe Yiras Shamayim is how much your are internally aware of Hashem in your life, and act accordingly. It’s what you do when “no one” is looking — except for Him. Many people know how to “walk the walk and talk the talk” — Yiras Shamayim is what is left when you strip away all of the social pressures and supports.
On a side note, I once heard from a Rosh Hayeshiva that he was once asked about a boy’s yiras shamayim when someone was researching a shidduch. His response was that Yiras Shamayim is a tremendously important thing in a shidduch — but how could any one else possibly know that about a person!!