(i'd say the same applies for teams, except that they don't need one specific player who is the leading authority).
Here are possible criteria. Let me know if i've let something out that is important.
- it just needs a founder/leader/admin, not members (1337) - Yes/No
- 2 members (e.g. Naked Hippies) - Yes/No
- 4 members (classic ctf squad) - Yes/No
- 6-8 members (fort team) - Yes/No
- It has (had) more than 8 members - Yes/No
- it takes part in tournaments/wars with more than 2 players per team - Yes/No
- it doesn't have to take part in anything to be a clan - Yes/No
- it hosts servers (NoNerdsJustGeeks) - Yes/No
- it hosts tournaments - Yes/No
- their forums have a short url - Yes/No
- its players play usually in those servers that are used for wars (sumo/fort/ctf/hr/df etc) - Yes/No
- its tags are original - it's not some short-lasting revival (NW, pNd) - Yes/No
- a clan with more than 50 active players isn't really a clan anymore but an own community, isn't it? (there wasn't one yet i think...maybe SP is getting there ) - Yes/No