The Champaign County Clerk's office has posted photos of what vote by mail envelopes will look like. The envelope you receive will have dark blue vertical stripe on the left side; the envelope you drop off or mail back will have an orange vertical stripe on the left side. Images are in the tweet below.
Double check that you're registered to vote and have indeed requested a vote by mail ballot on the Champaign County Clerk's website (now newly redesigned and much, much easier to navigate).
Early voting begins September 24th.
Top image from the Champaign County Clerk's Twitter feed.