I mean, I waited at the Combat Zone because things seemed different with the 3 prisoners tied up at the front, plus the fact there was speech beyond the door that I was listening to, and I was sneaking.
But out and about I typically don't hesitate to kill an obvious raider/enemy. Plus I often snipe first, so on most ocassions I might not be in range to see that they aren't hostile.
That said, I open VATS a lot just to confirm an enemies position and that tells me if they are hostile or not.