We slug it as opinion.
Seems the accompanying driver's license-looking pic of my mug helps, too.
Adding to Azrael's comment above, though ... if by then the reader cannot figure out by his/her own accord that it's a commentary piece, I question their overall intelligence.
As it is, reader response columns based on knee-jerk reactions can be a lot of fun.
Free associating here, but those of us old enough to remember Eric Sevareid might recall his commentaries on the Cronkite broadcast were slugged as such. Didn't watch Howard K. on ABC or Brinkley on NBC as much. Anyone remember if ABC and NBC did the same?
Anyway, if the old rules applied today, the opinion slug would almost need to be a permanent chyron.