The evidence is based on an analysis of the eyewitness video. For example check out this video analysis
https://youtu.be/5em7GcDTg8U
What it shows is the taser cartridge is being dragged behind Scott. As Scott gets further away from Slager we see the wire become taut.
Why is this significant? This tells us that the taser barbs were in Slager's uniform - i.e. this is evidence that Scott shot Slager with the taser just moments before Slager fired his weapon (BTW if you look at the video closely you will notice that as Slager is beginning to raise his arms to fire his weapon Scott is actually partially facing Slager).
Other close-up analysis shows what appears to be a taser barb in the officers left ankle and in still photo's taken after the incident the officer has his left pant leg raised as if to inspect for injury (the barb obviously did not penetrate is skin as he would have got a shock).
http://i.imgur.com/fkNOcRj.gifv?_=1