The Near-Death Experience scale was developed in 1983 by Dr Bruce Greyson in an attempt to provide a reliable, valid and simple way for clinicians to identify an NDE.
It consisted of 16 questions, and the person would choose from a series of provided (weighted) responses for each one.

Link to Bruce Greyson’s original 1983 paper presenting the scale: The Near Death Experience Scale

In 2020 a paper was published by Charlotte Martial and colleagues, re-evaluating and amending the scale. They added 5 new content items to match characteristic experiences and reworded others.
The new scale was called the Near-Death Experience Content (NDE-C) Scale. Dr Greyson co-authored the paper which you can read here.

Evaluations of this new scale found it to be a “psychometrically-sound self-report instrument for assessing NDEs although the international association for near-death studies (IANDS) has suggested returning some of the questions to their original content & meaning to maintain continuity across studies.

The Near-Death Experience Content (NDE-C) Scale:

We would like you to answer the 20 following statements according to your feelings and thoughts at the time of the experience (not before, nor after) by choosing the answer that seems the most appropriate (only ONE answer by statement is allowed).

Every experience or sensation varies in intensity, which is why we would like you to specify the intensity of your experience using the rating scale (from 1 to 4) described below for each statement. If, on the contrary, you did not experience the phenomenon described in the statement, please choose ‘0 – Not at all; none’. If you happen to have experienced several times the same phenomenon during the experience, please answer by considering the most striking phenomenon.

Rating scale:
0 – Not at all; none
1 – Slightly
2 – Moderately
3 – Strongly; equivalent in degree to any other strong experience lived so far
4 – Extremely; more than any other time in my life and stronger than 3

1. Your perception of time was altered
2. Your thoughts speeded up
3. You heard one or several voices which did not have any material incarnation
4. You had the feeling of suddenly understanding everything about yourself, the others and/or the universe
5. You had a feeling of peace and/or well-being
6. You felt a sense of harmony or unity, as if you belonged to a larger whole
7. You saw or felt surrounded by a bright light without any determined material origin
8. You experienced unusual sensations (sight, hearing, smell, touch and/or taste)
9. You were aware of things beyond what your senses can usually perceive
10. You gained insightful knowledge about the future
11. You had the impression of being outside of, or separated from your own body
12. You had the sensation of leaving the earthly world or of entering a new dimension and/or environment
13. You saw or relieved events from your past
14. You encountered a presence and/or an entity (who might be deceased)
15. You had a feeling of non-existence, of being in a total void, and/or of fear
16. You came close to a border and/or point of no return
17. You made the decision, or were forced, to come back from the experience
18. You had the feeling of dying and/or being dead
19. You saw or entered a gateway (for instance a tunnel or a door)
20. You sense that the experience cannot be described adequately in words

Scoring the NDE-C:

A score of 27 or above (out of a possible 80) is indicative of a NDE, although the authors warn: “This is relevant for research purposes; however, from a clinical perspective, we would like to stress that each self-reported NDE (including individuals with a score below this cut-off value) should be considered. NDEs probably outline a continuum of experiences that are more or less rich in terms of content and it should be noted that no specific feature appears in all NDE reports, as found in many other studies.”