I know this has been a hot topic as of late and people in this sub have been trying to figure out how to deal with them. I spent about 15 mins looking at a few different subs and found some pretty interesting info. There are many ways to be a paid shill, but if you do just a little digging you can easily spot them. Here's some rules I've found help, it's not a concrete theory but a hypothesis.

1. Posters must gain credibility: most of these shills take it to the extreme. An incredible amount of posts over a short period of time.

2. Not always the case but short account life span

3. Most users have specific habits ie: they frequent certain sub's, shills tend to crawl, many different topics, only one or two comments on a certain post.

Below I've posted some profiles that raise suspicions:

http://i.imgur.com/XdOeyxV.png (high karma 1 month life span)

http://i.imgur.com/jQNves5.png (post crawling too)

http://i.imgur.com/9YNwwyk.png (Reddit must be a full time job)

Normal looking account:

http://i.imgur.com/scS35OX.png

I'm not implying that the above accounts are paid shills but there are definitely some odd metrics associated. If anyone else feels like digging or has suggestions for spotting these shills please feel free.

Another thing I've noticed with these high activity accounts is the attention to grammar and punctuation. It seems like their posts and comments are being proof read for errors.