A true heel should be universally hated by most. I'd consider Jinder and Corbin the closest characters to a true heel due to:
A. Have been heels their entire career
B. Get consistent heat when pushed
Heels that get cheered are not true heels to me. For example, Elias, Owens, Sami Zayn, Johnny Gargano. These are heels the fans secretly want to cheer and want their validation. They are witty, sarcastic and almost in a way similar to Jericho but I don't know how else to put my finger on it. They all seem like tragic comedy characters.
Also, unpopular opinion but guys like AJ, DB and other underdog characters should be used SPARINGLY as heels because it doesn't work, at least not to me. DB turned heel for no reason it seems other than they wanted him as a heel. When it's actually required for the integrity of the show (Roman, Cena) nothing happens.
Also, WWE don't seem to care much anymore about fan perspective when switching characters alignment. Rusev, Strowman and Elias are perfect examples of that.