Same here. I first saw an enemy player on Nar Shaddaa and his name was in blue, like mine, so when the first time I saw an enemy player out in the field with his name in yellow, I paused and scratched my head wondering why it was different. Then I remembered that some mobs have yellow names, which means they aren't openly aggressive to you, but they CAN be targeted and attacked. That's how I realized he was flagged for pvp, and the first thing I did was immediately toggle off all my companion's aoe abilities...then kept a very close eye on where that enemy player was while I continued with my quest.
This was on a pve server. It was annoying to HAVE to do that, but it did save me from potentially getting griefed.