Sick of typing "/script d(GetActiveCompanionRapport())" in chat, but don't need or want anything more complicated?
This no-frills addon is for you. ;)
Install it by popping the whole (unzipped) folder in
...\Documents\Elder Scrolls Online\live\AddOns
...\Documents\Elder Scrolls Online\live\AddOns
Type "/rapport" in chat, and see the rapport for your current active companion. That's it.