The signs of a cult are:

1. Belief that the group has special knowledge.
2. Secret beliefs. Example mormons.
3. Encourage / prohibit members from interacting with non members.
4. Shunning if you disagree.
5. Strong or charismatic leadership.

My safety briefing on cults:

Whatever you do, bring your own Kool-Aid so you don't die! (Just in case you change your mind and want to leave.)?

My definition of a cult

I prefer not to use the word cult as there are too many differing interpretations of what a cult is and whether or not the word should even have negative connotations.
Cult is one of those odd words that has a mixed etymology.
True cult can be assumed come from the same root Latin word cultus as does culture and cultivate, but over time it has also become associated with the Latin word celare the completely unrelated root word from which we gain occult, it means hidden or concealed especially when applied to "sacred" knowledge, hence the sinister connotation. However since it is a convenient word, in is in wide colloquial usage I will hereafter define the word as I understand it and in all further uses of the word cult by me, it will be in this stated context.

1. A cult is an individual or organisation that promotes the idea that evidential truth is of less value than an act or matter of faith, even and especially when that faith is directly in contradiction of the proven facts available.

2. A cult is an organisation or individual that ascertains a following for reasons of personal gain and aggrandisement by exploiting the fears beliefs and weaknesses of other, by deception, misrepresentation and specifically manufactured fictions presented as facts and objects of devotion.

3. A cult is an organisation that uses the threat of spiritual, emotion and social distress to excerpt an undue influence over the life decisions of its membership for the purposes of retention and recruitment.

4. A cult promises a progression through a set hierarchical. Spiritual, emotional, social or physical developments as dictated by levels of participation, contribution and duration in membership of said cult. These progressions available only by a greater and great degree of self-abnegation and abasement surrendering greater and greater control of life decision to the cult for the purposes of “betterment”. In return, each progression will involve the gaining of new and hitherto secret or sacred knowledge or the right to possession of some form of sacred object, talisman or form of dress.

5. A cult promotes the idea of faulty thinking and unworthiness in its members when the cult itself fails to provide the promised state of well-being that membership implies. In order to encourage deeper and more active participation, re-education, acts of faith or greater contribution enable the promises of greater well being to be reiterated reliant upon the attainment of correct thinking and greater worthiness.

6. A cult promotes the idea of internalisation by isolation from the profanity of contaminated “other”. To this end, it promotes the ideal that only families constructed within its rank will be happy or good and the rejection of even blood relatives or spouses who are “other”.

Many religions will fit with one or two of this indicators but only organisations such as the Mormons, Moonies, Scientologist and a few others generally defined as cults have at least five indicators.

In short key indicators of a cult are

Undue influence on the private life of the individual
Conformity of dress and behaviour
Suppression of individuality
Obsessive and over protective devotion to an object, individual or organisation
A reluctance to hear or tolerate counter arguments to the "official line"
Isolationism of the individual from "profane" influences

When this comes about the organisation concerned appears to be religious, even if there is no supernatural or spiritual element involved (See Scientology or employee of the Disney corporation)

I have noticed that control and coercion often takes the same forms regardless of whether it’s a cult or a fascist government or an abusive partner. Isolation, lies, gaslighting, playing the victim, asserting that everyone else is lying about us/me, fear/threats.

Membership seems voluntary but people are expected to cut ties with their old life and their family members. Once a member, it becomes difficult to leave. Cults are coercive and attempt to emotionally control their members to keep them in the group.