Knows how much any fish, collard greens, turnip greens, peas, beans, etc. make up "a mess" (as in "a mess" of greens).
Only a true southerner can show or point out the general direction of "yonder."
Only southerners know exactly how long "directly" is ... as in, "Going to town, be back directly." (Generally pronounced "dreckly")
All true Southerners, even babies, know that "Gimme some sugar," is not a request for the white, granular sweet substance that sits in a pretty little bowl on the middle of the table.
All true Southerners know exactly when "by and by" is. They might not use the term, but they know the concept well.
A true Southerner knows that "fixin" can be used as a noun, a verb, or an adverb. (As in, I was fixin' to go over to BettyLou's. Or, we had a huge Christmas dinner with all the fixins. Or, are you fixin' my car next?).
Only true Southerners make friends while standing in lines. We don't do "queues," we do "lines," and when we're IN, not ON, line we talk to everybody!
True Southerners never refer to only one person as "y'all" ... more than three is way more than one, it''s "all y'all."
Only true Southerners say "sweet tea" and "sweet milk." "Sweet tea" indicates it contains sugar and lots of it ... we do not like our tea unsweetened. "Sweet milk" means you don't want buttermilk.
And a true Southerner knows you don't scream obscenities at little old ladies who drive 30 MPH on the freeway. You just say, "Bless her heart!" and go your own way.