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.