How do you know if a group is a sect/high control?
I've never come across a sect or high control group which admits to being one. Its just not what they do, you have to work it out for yourself. These groups do exhibit a series of behaviour flags though, which will alert you to that fact.
I often get contacted by people concerned about relatives or friends involved with unusual groups, asking 'is it a cult?'
Here are some tips:
Check out Mormonstories.org for an assessment tool. They are a podcast discussing Mormon and ex Mormon stories. Obviously, Mormans are only one of a wide variety of these groups - the stories may be different to your own/friends/family experiences - however its useful to hear other peoples stories of exit and recovery.
I find the assessment tool useful as it covers a wide array of high control tactics. 29 of the common high control tactics are listed below.
Dispensation of existence
Destabilisation of Self
Black & White Thinking
Us v's Them
Doctrine over Self
Belief follows Behaviour
Emotion over Intellect
Identification and Example
Guilt and Shame
No Honourable way to Exit
The Atlantic has an article - 7 Signs that you're in a Cult - which is a good read.
Dr Steve Hassan is a leading American expert in the field - he also has some interesting resources on his website. He has a BITE model of control which outlines some high-control tactics.