This is an interesting idea. Here's a quick proposal for how to implement it. 
Automated tweets would be marked with a robot icon. The marker is at the tweet level rather than the account level because some accounts mix automated and organic tweets.
Some bots are spammier than others, and unless one searches through the user's tweets this isn't always obvious. Repeated automated tweets would thus be marked with the number of times the tweet in question had been posted alongside the robot icon.
