Very few people ever set out to become cult members on their own. Regardless of how vulnerable people may be, if they do not meet a recruiter there is little likelihood of their 'joining' a cult.
Question People with easy answers of solutions to world problems. Whilst some easy answers can be found, things may not be as they initially appear.
Question people who are excessively or inappropriately friendly. There are few instant friendships.
Question people with invitations to free meals and lectures, where the objectives are not clearly stated. There may be a hidden agenda.
Question people that pressure you because "everyone else is doing it". After all "everyone else" could be making a mistake.
Question people that recruit you through guilt. Guilt induced by others is rarely a productive emotion.
Question invitations to seminars having nebulous goals. There is no reason to be vague or evasive, unless there is something to hide.