Here is a worksheet on differentiating the use of genius and brilliant. This document, like all documents under the title above, are informed by Paul Brian’s book Common Errors in English Usage, to which he allows full access at no charge at the Washington State University website.
The simple usage point of this worksheet consists in this: genius is a noun, and brilliant is an adjective. In other words, you can say “He is a genius” (using genius as a predicate noun), but you cannot say “He does genius work,” because genius is not an adjective and cannot modify the noun work. You can also say something like “I think Sergei Eisenstein’s Battleship Potemkin is a work of genius.”
You can, however, say both “He is brilliant” (using brilliant as a predicate adjective), and you can say “He does brilliant work,” (using brilliant as an attributive adjective). You cannot say “He is a brilliant.” It doesn’t sound right in any case, though, does it? Brilliant, as above, is an adjective and should be used as one.
If you find typos in this document, I would appreciate a notification. And, as always, if you find this material useful in your practice, I would be grateful to hear what you think of it. I seek your peer review.