It's not always obvious nor easy to know what's going on during that time, especially for a dying person who might have been very tired or scared or that kind. Because, many might be the reason: it might be caused by psychology estate of the person, or just his strong belief or genuine.
Just saying that all of them are fake is not wise, but saying that all of them are genuine is carelessness which might lead into error.
So for me, I can't say for sure, but I know that it's a good sign if he says that angels or Jesus is smiling at him, because at least he is looking forward for the blood of Jesus who paid for his sin's wage.
Seeing demons, or being scared is sign of lack of faith if not genuine, so what you can do is pray, and remind him that Jesus was dead for him, just say it loud, who knows if he hears it and repent in his spirit before he dies.
Note: No one knows for 'sure' who will go to heaven. Just know yourself as you check all the time if you are in faith or not.
2Co 13:5 Examine yourselves to see whether you are in the faith; test yourselves. Do you not realize that Christ Jesus is in you-- unless, of course, you fail the test?