Domain terraforming involved orbital mirrors/shades which are incredibly durable.
If any worlds you find don't have terraforming infrastucture then what you see is likely the 'natural' state of that world.
The useful exception to this being Asharu. As this world was previously terraformed to some extent, and then de-terraformed when the infrastructure was kicked out of orbit. So we don't actually know what either of it's previous states were (unless it turns up in lore), but we can still see the now rogue shade still in orbit of Corvus.
Asharu 'proves' that if the mirrors/shades were to be removed either deliberately or accidentally, the world beneath would revert, and change to a condition closer to it's original form.
So a terraformed Terran world could not remain Terran without that infrastucture.
Thus any Terran worlds you find in the sector which do not host orbital mirrors/shades are very likely not the product of terraforming.
Yes, I do spend an impressively wasteful amount of time contemplating planetary vandalism. (That's what terraforming is really, when you think about it.)