I already know the commercial trackers and calnexin, calreticulin, and KDEL, but I was looking for a marker more specifical of the membrane itself, and not of its lumen, neither signal peptides.
You could use sarco/endoplasmic reticulum calcium ATPases (SERCA-type calcium pumps).
Calcium is accumulated in the ER by sarco/endoplasmic reticulum calcium ATPases (SERCA-type calcium pumps). Located in the ER membrane, these enzymes translocate calcium ions from the cytosol into the ER lumen by active transport, dependent on energy obtained by ATP hydrolysis.
Please refer to the research article below for more information.