Sometimes white tips are a sign of trouble but I see white tips when corals are growing too. How can you tell if your white tips are a sign of trouble or just growth?
I think white growth tips won't spread down the branch, they'll stay relatively the same size as older growth colors up. I think generally too, you'll see the growth tips on most of the branches, whereas if it's RTN/STN it would be more localized. I'm far from a SPS expert though.
If they are uniform and show a slight tint of the normal coral color (like super light pink on a red coral), then growth... usually. You should also just be able to tell that your corals are growing - take some photos to keep around for reference.
It takes a while for the zoox to populate the new growth areas, which is why a lot of new growth is a different colors... it is not always white and sometimes it is purple, green, etc. Some growth is the same color as the coral. This is highly variable.