Photo of a white wallaby
A wallaby and a kangaroo are similar in appearance but are as different as a pony is to a horse. If you think about wallabies as being the ponies of the marsupial world and the kangaroos as the horses then you will have a good indicator of how to separate the 2 species.
Wallabies have a much shorter and compact appearance while the kangaroo has the more muscular build along with the long legs. Kangaroos can stand as tall as 7 feet when fully upright, where as the wallaby is only 3-4 foot tall.