Is a skill check always an action in structured gameplay (combat)? I was looking at faq/errata and noticed how Piloting can be used as an opposing check to determine the relative facing of two ships - example provided is a fast ship attacking a walker - Thus my question: would that be an action in itself, or could it be just a "reaction" (or free check) which would allow the pilot to attack in that turn?

As I read it, a skill check is always an action except for when a GM calls for one outside the normal flow of structured gameplay. So in that example, I probably wouldn't count it as an action.

I think exceptions are generally called out specifically. For example, in the Age of Rebellion squad rules, you form a squad with a Leadership check as a maneuver.

I think it was on the Order 66 podcast that Sam Stewart said that he would, in some circumstances, have someone piloting a ship in tricky terrain make a Piloting (space) check as part of the Fly/Drive manoeuvre. So there are exceptions. But most of the time skill check = action.