According to [audoban.org][1],

>Ravens often travel in pairs, while crows are seen in larger groups. Also, watch the bird’s tail as it flies overhead. The crow’s tail feathers are basically the same length, so when the bird spreads its tail, it opens like a fan. Ravens, however, have longer middle feathers in their tails, so their tail appears wedge-shaped when open.

[According to allaboutbirds.org][2],

>One of the best ways to tell crows and ravens apart is by their calls. 

One this [page][2] you can find recordings of what each sounds like.


  [1]: http://www.audubon.org/news/how-tell-raven-crow
  [2]: https://www.allaboutbirds.org/similar-species-crows-and-ravens/