1. They have a charismatic "infallible" leader. Hero worship.
2. They ask you for money.
3. They do not want you to tell other people what their 'teachings' are. Secrecy.
4. Groupthink. Thoughtcrime. Unquestioning acceptance of teachings.
5. They have a fixed system of teachings that you may not question or doubt, and if you're allowed to "question" the teachings, the ultimate conclusion is that you're wrong and the Leader is right. Even if questioning is tolerated, ultimately you're "led" back to the "truth". Drink the Kool Aid.
6. They try to substitute your family and friends for other members. Closed social circle. If interference by "outsiders" is detected, those "outsiders" are "encouraged" to "experience it for themselves" or they're treated as enemies, and you're encouraged to think of them as such.
7. You're discouraged from leaving, either by emotional blackmail, or direct threats of excommunication or refusal to be allowed 'back', or worse. Ostracism.
This description describes some MLM companies, political groupings, some religious organisations, and some self-help organisations. I won't name them but you know who I mean. If it checks all seven, it's a cult. In fact, early Christianity as depicted in the gospels matches all criteria except (2).