A classic sign (sufficient, not necessary) is that they are extroverted but surprisingly bad at connecting with strangers when alone. In fact introverts do better than them. But with 2-3 clique members for support, they deal beautifully with strangers, making smooth in/out decisions. This is because they rely strongly on peer cues to make such decisions. This is true even of the alpha.

About 2 in 3 cliquey types present this way, so it is a good test. The remaining 1/3 require more sophisticated tests.

A simpler but less reliable test is watching cellphone use in public settings when they are alone. We all do it these days, but cliquey types do it more obsessively and for longer periods. There is a danger of false positives here though. The person might simply be actually busy, or bored by the surroundings and reading something. Look for a lot of scrolling and frequent tapping/typing that does not have the email or document reading tempo. Sneak a look and check if it is Facebook or IM or texting to increase certainty.