Since ATF6 high levels of mRNA and spliced forms of protein used as marker of ER stress, ATF6 is a good candidate to check for protein folding and UPR.
A specific marker for protein folding stress is increased protein expression of the chaperone BiP/Grp78. ATF6 is a bZip transcription factor with gene targets that encode cellular processes involved in responding to ER stress and the unfolded protein response, but it is not directly involved in folding per se and is not a direct marker of folding capacity.