That is my understanding, and this is what I do on my airplane.

An easy way to do it, is just go to "reports" and insert pilot's U number. Get his / her seniority number and then do the same with the other pilot. If there is an issue or a concern beyond that I would call the flight office and ask a person there to look it up and tell you who is senior. I would hate to bother the FODM with something like this. But, the reports portion of the CCS website seems adequate and quick. And, I would politely ask the more junior of the two jump seaters to do it for you and ask him/her to show you what the results of the querry are.