The word "CERT" is printed in bold on the boarding pass and sometimes the check in agent will also overwrite the word "CERT" again using a marker pen across the boarding pass, presumably to alert the gate agent to pull the cert. but also in a way to tell the FA that you are second class J passenger, you're using a cert.

When I web check in, I always print 2 boarding passes and throw away the one marked up by the check in agent so that there is only the smaller machine printed "CERT".

AC also likes to change equipment at the last minute. Upgraders will be amongst the first to be bumped if the replacement aeroplane has fewer J seats than the original one.