What is the easiest way to differentiate between a fairy, a pixie, a sprite and a fae in fantasy novels?
Fae = a general umbrella term for all non-evil non-human but distinctly humanoid creatures out of English or occasionally French (but not Irish or Welsh) lore. In modern writing, it can expand further to encompass some non-evil non-human humanoid creatures from German, Slavic, or occasionally even Middle Eastern or Japanese lore.
(Note that non-evil does not mean good. It does not even mean "not cruel." It just means they are not distinctly evil (such as demons) and do have the capacity to be good. However, they very often are depicted as being dicks to humans in one way or another, but usually not in a manner that is filled with distinct malice or murderous intent.)
Fairy = a general umbrella term for winged creatures that have an otherwise human-like appearance. This definition makes no differentiation on size, but those who are full normal human size are generally depicted as far more powerful. There are also some that can hide their wings, or their wings are naturally hidden unless manifested magically. Their wings are always depicted as being insect-like, but there is a wide range of possible appearances within those standards. The most common depiction is butterfly-like, but a dragonfly-like arrangement is also common. There are also depictions that involve the fairies having wings of light (often in the "not visible unless magically manifested" varieties.)
Pixie = a winged humanoid specifically noted for their small size, usually of a size that can fit in the palm of an adult human's hand, ranging from the size of a doll at the high end to the size of an adult's thumb at the small end. Their wings are almost always depicted as being dragonfly-like.
Sprite = a very poorly defined term that sometimes has overlap with "pixie" in it's definition. Can also, in some cases, drift into the realm of describing immaterial beings or beings of pure light and energy who are formless in nature. However, even when going into that territory, they are always depicted as small and never depicted as individually powerful. (Can be incredibly powerful in numbers though.)