you can use is_member_of() function...
Code:
is_member_of()
Works out if a user is a member of the specified usergroup(s)
This function can be overloaded to test multiple usergroups: is_member_of($user, 1, 3, 4, 6...)
boolean is_member_of (array &$userinfo, integer $usergroupid)
* array &$userinfo: User info array - must contain userid, usergroupid and membergroupids fields
* integer $usergroupid: Usergroup ID to test