One of the great democratizing forces on Twitter is getting a concise look at the stats someone's followers and who he or she is following. When you're not sure who a person is immediately, there are some heuristics you can use to figure out if they're worth following.
A dead giveaway is when someone's following a ton of people and few are following back. It's okay if there's a decent disparity - a 2:1 follower:followed ratio is perfectly normal. Yet once you start getting into 3:1 or definitely 4:1 territory, something appears off.
Would you ever follow a Twitter user with these stats?:

That's 12.5:1 follower:followed. Sure enough, the messages reeked of twitter spam. Is there any scenario where you've followed someone with numbers like this?
An aside: I'm trying to get into Twitter more these days; follow me and (assuming you're not a spammer, psychopath, or former governor) I'll follow you back.
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About the author:
David Berkowitz is Director of Strategic Planning for 360i and oversees the Search Informed Marketing firm's Emerging Media Practice. Every Tuesday, he pens the Search Insider column for MediaPost, with over 100 articles published to date. He often speaks at events covering marketing, media, and technology; his previous engagements include Ad:Tech, Consumer Electronics Show - Digital Hollywood, MediaPost's Search Insider Summit, and many others. He has also blogged extensively with
MarketersStudio.com, MarketingVox, nowEurope, AdTechBlog, and others.