From my experience no AHJ has required underground conduit to be labeled but where it is exposed, yes. We mostly use "Caution Solar Circuit" reflective stickers. Not sure there is a code rule on the underground conduit being labeled or not, I just have never been called on it.
Not an NEC requirement. The NEC only requires labeling inside a building. Another code might require it; for what it's worth, I haven't seen a requirement that applied to ground mounts.