Until one day I found a stack of cheques in a drawer, thinking I had hit it rich in forgotten to cash cheques only to find out I had already deposited them all with my phone.
that's why I always take a pen and put a small mark across the top left corner of the check. It doesn't obscure anything on the check, and I can tell it was mobile deposited. I do this AFTER I scanned the check into my phone.
I use an iZettle reader that connect to their iOS or Android app. They are kind of the European version of Square. They do have a complete POS system with receipt printer and cash drawer, but I don't use that since I don't have a store-front.
Available in quite a few countries, but doesn't look like the US is one of them yet.