As a new world blackbird, the Rusty blackbird is a close relative of the Common Grackle, Red-winged Blackbird, and Brown-headed cowbirds, and has a structure similar to these birds. Identifying it is usually done in comparison to its other close relatives.
The Rusty blackbird is more distinctive in some plumages than others. The male in breeding plumage is black overall, with a pale yellow eye, and is best identified from other all-dark, yellow-eyed blackbirds (such as the common grackle) by size and shape. In winter, the male has significant "rusty" color on it. The female, in non-breeding plumage, is brownest of all, especially on the back and crown.
Most blackbirds are easy to tell apart from the rusty blackbird. The trickiest to tell apart are the Brewer's blackbird: Brewer's blackbirds have a very similar build and structure, and female and juvenile birds of this species have a similar brown coloration. The two different in habitat: Brewer's blackbird frequents grass, whereas the Rusty blackbird favors swamps, especially those in or near forests.
Cornell Lab of Ornithology has excellent ID tips for the rusty blackbird on their site All About Birds.