Some groups try and wiggle out of the cult label by saying they're not a cult cos they're not secretive about what they do but the key is - are they a totalist organisation? So for example, the labour or green parties are a broad church, you can just be a member and put your cross in the box at election time or you can be very involved at local level, leafleting, door knocking, involved in community initiatives, standing for election, the full shebang. But you're both members of the party and it doesn't have to leak over into other aspects of your life. It doesn't dictate how to be in a relationship, how to educate your kids or what characteristics to cultivate in your life. It doesn't tell you what to think beyond expecting you to generally agree with the aims of the party but there's a great deal of leeway within that too. You can be further to the left or right of it. You can try and push for more progressive policies or urge caution and a more central stance. You can disagree with those in leadership roles.
None of that is possible in a cult. It is a totalist organisation that will demand or forbid behaviours. It is absolutist. There's no personal interpretation allowed. There's no questioning of leadership. And where there isn't a named leader/s, the organisation itself will be in that role.
Something I've found helpful in distinguishing between cult and crackpot but harmless fringe groups.