Comments are busted at Ms. Aniston’s blog (it complains about a missing captcha, except there’s nowhere on the page to enter it), so I’ll throw my answer to her question here:
One possible method of determing class is to use station in the hierarchy as a metric. If you have a boss, and you have people who call you boss, you’re middle. If you only have those who call you boss, you’re upper. If you only call others boss, you’re proletarian.
Of course, that gets murky when you’ve got “team leader,” and journeyperson/apprentice situations, which comes back to the IWW’s metric: can you fire others? If so, you are a boss. Perhaps not the boss, but certainly a boss. If you cannot fire others, you are not. I should probably note that I’m not in favor of simply granting small business owners a pass. Ultimately all small business owners are simply large business owners in an infantile state—those who do not aspire to be billionaires are on the way to selling their business to those who do. I liked Trotsky’s take on their role (and danger) in Fascism: What it is and how to fight it.
James Cape 
There’s a subset of small business owners. People who can’t or won’t make themselves employable in the conventional structures. They tend not to be able to offer employment either, at least not regularly. Most of them live in rural or the less affluent urban areas. They’re ineligible for union membership, by virtue of occasional hiring and/or the legal status of the business. The Ma and Pa stores are good example. The guy who owns a lawnmower, a weedwhacker, a chainsaw and a pickup truck is another. They’re not looking to get rich. They just can’t bring themselves to call someone “boss” every day. It’s the American Dream (now with protected bankruptcy, the most common fate, almost completely out of reach and karoshi claiming many of the rest).
We’re going to have a whole class of effectively criminalized petit bourgeois soon. They, along with the downwardly mobile, formerly employable middle class, form the recruiting base for a populist authoritarian movement, and sometimes state sponsored domestic terrorist groups. Just like Trotsky says.