Benefits in Kind - taxable/non-taxable

Anyone knows the simple self-made guide how to identify what benefits in kind are subject to either NI only, NI+tax or no NI or tax? I know there are questions to ask like, who organised the benefit and who paid for it, was the cost reimbursed etc. But there must be a key criteria which would give an indication straight away. I just want to be able to recognise it immidiately.  For example, employees comes back to directors asking why their tax code changed and for some they never did. Then directors come back to us and it can be a bit confusing even for us. Thanks in advance!