Doing LE & security details, I can honestly say there can be "cues" or behaviors that a trained person can pick up on re: EDPs.
The NYPD's elite ESU(Emergency Services Unit) even has specially trained cops to address mental health & unstable subjects in critical incidents.
A person can tell if someone is going to harm themselves or others.
I'm not saying you need to be Dr. Phil Magraw or Drew Pensky with everyone you come into contact with but there are behaviors you can spot or be aware of; crying, lack of focus/easily distracted, paranoid or fearful behaviors, talking about people or events in the past tense, sweating a lot, pacing or erratic movements, wierd or inappropriate humor/questions, mood swings, lunges or confrontatational stances when speaking, flushed appearance or reddened skin.
As I posted, these may or may not be present but if they are or you feel unsafe on a gun range, then ACT on it. If no safety officers or staff are there, then leave & contact the non-emergency number/soc media system of the area's sworn LE agency. They can deal with the person & get them mental health treatment if needed.