The difference between a white moth and a white butterfly is in their antennae which in white butterflies they have clubbed tips, and for white moths the antennas have feathery/saw edged tips.
Also other differences between a white moth and white butterfly is in how they hold their wings when at rest.
White butterflies hold their wings vertical at rest and white moths hold their wings "tent like".
And white butterflies are also active during the day and white moths are active at night.
White butterflies also have a slender body shape and white moths have fuzzy and stout body shapes.
Although some white moths, such as the Cabbage White moth, which is a butterfly, break these rules, which makes the antennae the most reliable indicator.
White butterflies have antennae's which are thin, long and have a distinct club or bulb at the end.
And white moths have antennae's that are feathery and comb like or saw edged without clubs.
White butterflies hold their wings vertically and closed above their back when at rest.
And white moths hold their wings flat or folded in a tent like shape over their abdomen when at rest.
White butterflies have slimmer and smoother bodies than white moths.
And white moths have fuzzier and more robust and stout body's which are covered in fine hairs.
White butterflies are diurnal and active during the day.
White moths are mostly nocturnal and active at night.
For the pupa stage of white butterflies they form a hard chrysalis.
And white moths spin a silk cocoon.
Many white butterflies like Cabbage White butterflies are also common in the daytime, but if you see any white, fuzzy insects with feathery antennae's flying at night, then it's most likely a moth and if it's has a clubbed antennae and flies during the day, then it's a butterfly.