Humans have a "reserve" of willpower which can be depleted. This is science; I can't find the study right now but here's an article about it?
http://www.nytimes.com/2008/04/02/opinion/02aamodt.htmlNow, consider actions which you take. Assign actions to depleting or filling up your store of willpower.
Do you find that various forms of social activity deplete or refill your store of willpower? If all the social activities you can think of deplete it, then you can safely assign yourself the introvert category.
Anyway, I'm not actually sure that this exercise is valuable. it seems like you might rather keep your identity small (see PG essay on the topic), as it can only constrain you.