In southeast Louisiana, it is common to see large white wading birds- in wetlands, urban drainage canals, even stalking peoples' yards. Three common members of this group are the Great, Snowy, and Cattle Egrets. These three species are easy to tell apart based on two characteristics- overall size, and beak color.
Great Egret- very large (chest high on a human in typical posture), yellow beak.
Snowy Egret- small (thigh high), black beak.
Cattle Egret- small (knee-thigh high), yellow beak.
There are other differences that are also definitive, if seen. Cattle Egrets often have a buffy tinge on the crown and breast (especially in breeding season). Snowy Egrets have black legs (often with yellow stripe up the back) with yellow feet. The egrets' shapes are noticeably different to the trained eye- Great Egret long and slender, Snowy dainty, Cattle squat and short-necked. Great Egrets have a characteristic "leaning forward" pose with neck and beak extended diagonally upward, that gives away their identity even at a great distance. Cattle Egrets are often seen in flocks of a half dozen or more on short dry grassy surfaces such as the margins of roads- though the Great often hunts away from water, you will seldom see more than one or two in the same place doing so.
There are two other white waders with which egrets might be confused. White Ibis are common, even in urban areas, and have conspicuously down-curved beaks; they fly with neck stretched out (egrets tuck theirs) and have black wing tips. During their first year, Little Blue Herons are white. Young Little Blues are less frequently encountered than any of the three egrets, but are not uncommon. They take some care to distinguish. They are the size of a Snowy, but have a blue-gray beak with a crisply defined black tip (Snowy's black beak can become pale toward the base, but it doesn't form a sharply delineated black tip). Little Blues have green legs and feet. Their shape is less dainty than Snowy Egrets- thicker neck and thicker beak. Many have a gray tinge on the wing tips in flight, which is diagnostic if present.
If you are at Grand Isle-Elmer's Island-Fourchon, you may also run into a white phase Reddish Egret. They are in between Snowy and Great in size, have a notably bulky/shaggy neck, and usually a flesh colored beak with black tip. No yellow slippers.
James Beck provided the photos below (Thanks, James!), which are in the order Great-Snowy-Cattle. The Snowy and Cattle are in unusually bright nuptial colors.