Sure.
(g)pu (d)isable all cards, then measure the remaining (in my case 81) Watts, and subtract that baseline everywhere.
It's not exact, an idle card still uses a few Watts, perhaps even only single digit - but compared to >200Watts, it's only ~5% wrong.
A discussion of that topic is in
posting #72. Come over if you are interested.
Do you know if there a thread for the R290X also, looks neat, and must try to optimize my rig soon... :-D