The necks are able to kink in an s-shape, due to the modified shape of the sixth vertebrae. The neck is able to retract and extend, and is retracted during flight, unlike most other long-necked birds. To differentiate itself from the Herons, all Egrets are white in color except one species, the Pacific Reef Egret has two color morphs, the light and the dark. The percentage of each morph varies geographically. White morphs only occur in areas with coral beaches.
Most Egrets are seen in open country with wetlands but gather in large numbers in the rice fields and mudflats. There may be isolated individuals staying under the shelters on river banks. Apart from the variance in size between a couple very marginal, it is a daunting task to make a positive ID in the field as judgment by size is neither easy. I will attempt to list out some method commonly used to differentiate them.
Focusing on Egrets that can be seen in Malaysia: