Mostly the same as mechanical stuff- look for any damage, make sure everything functions well, and you should be in good shape. If you want extra piece of mind, take it to shop that's competent with Di2 and have them connect it to a computer and make sure everything is reporting correctly and up to date on firmware. Clarifying what Marcus_Ti said, 7970 is the only one to really avoid, 6770 (first gen Ultegra, second gen Di2) uses etube, which means wires and other parts are readily available, and is bueno if you're ok with ten speed.