Check the quality. Do they look like they're carefully crafted to stand up to Chanel's standards of quality? If they look chintzy, that's your first clue. You shouldn't be able to pick up a pair and immediately find flaws in the design. That includes something being a little "off" with the logo (misspellings are common), or missing packaging.
Inquire about the return policy. If you can't return or exchange them once you buy them, something isn't quite right. If they're legitimate Chanel sunglasses from a reliable merchant, you will usually be able to make a return within a reasonable period of time.
Check for a warranty from Chanel. You definitely won't be getting any type of warranty from Chanel if you're buying fakes. A vendor could potentially fake the document as well, but if they don't offer one at all, this could be a red flag.
Get the exact model number of the sunglasses. If the vendor can't provide one, be wary. If you're shopping online, once you get the model number, you can check with other well-known sites to see if the model number and color scheme match up.
They should be made in Italy.