I did a campaign objective once that said that only crusaders in the second slot of each bench could be used. While the tooltip on a crusaders portrait says their bench number, it does not say which "slot" in the bench they are in. This was frustrating since I had at least 2 crusaders in nearly every bench slot, but in the majority of the benches all crusaders were locked out for the campaign. Apparently I had very few "slot 2" crusaders in the then 36 benches.
How can you tell what slot in a bench a crusader is in?
Ah yeah, looks like that objective was based on the global list of Crusaders and not the Crusaders you had unlocked.
The only way to tell currently in game would be to own all the Crusaders, to have the Crusader who originally unlocked the bench seat showing on the bench currently, and then the Crusaders listed on the Swap button would be sorted by order of release, so the one at the top of the list is the 2nd swap for that seat. But that only works if you own all the Crusaders.
That mechanic has only been used on that one event objective so far, and to pass that, you'd just need to check who was unlocked and available to use, as you ended up doing.
If we end up reusing that mechanic for more objectives in the future, I'll gate it on at least having completed the Tier 1 11th objectives, so you'd have 4 Crusaders available, and we could look into adding what number in the bench seat a Crusader is.