Above - Green-veined White
Above - Small White
I have taken these two photos in the last two days and noticed they are in a seemingly similar position.
In the past I have generally always separated the two species by looking at the very different underwings.
Apart from the difference in spots in this photo (two spots can be found on both species), you will notice that the black/dark tipping to the forewing is a single constant patch on Small White and is made up from broken parts on the Green-veined White. I noticed this feature today and subsequently discovered it is actually a really useful feature to split the two. You may already know it, but I previously didn't.