I can only really speak of the Jehovah’s Witness cult as I was raised in it. For the Witnesses, it usually starts at the door and is quite non-threatening. They spend most of their time in their Kingdom Halls learning how to be effective at the door and with their Bible studies with prospective members. Thus, they start off really innocent and small at the door and don’t rock the boat. They usually take a few Bible passages out of context and ascribe new meaning to the text to wow the prospect with their knowledge and to show them what the Bible “really teaches.” Other than that, they usually ask prospects if it would be nice to have a world without suffering and death, really innocent stuff at the beginning. If they told the prospective member everything they believed right off the bat, they wouldn’t be successful at all. If they get a Bible study, then they get a little deeper in Jehovah’s Witness beliefs, but they mostly keep away from the controversies at this point. If the Bible study is going well, they then invite them to start attending “meetings” (services) at the local Kingdom Hall. With regular attendance at the Kingdom Hall, their belief system starts to get flushed out more fully for the prospective member. Eventually what the Watchtower Society says starts to take over the thoughts and opinions of the new member completely.
It is a slow brainwashing process and should be a major red flag that beliefs have to be hidden until the prospective member is “ready” for them. I think most cults use this technique. With the rise of the internet, it has become much harder for Witnesses to witness and their retention rate is horrible.
Telling if someone is brainwashed or how brainwashed they are can be tricky. That said, if a person is devoid of most/all independent thought, that is a good clue that something is amiss. If a person has to check with their "superiors " to see how they should think or feel about something, another good clue. Lastly, if they reject facts/reason when it stares them in the face and become quite hostile, another good clue that something may be amiss.