Existentialism appeals to highly individualist people ("irreducible subjective") who believe that the core of their existence cannot be shared with the collective (hence "existential aloneness").

In a daily-life sense, "existential atheist" is kinda redundant since the symptom of both atheism and existentialism is anti-collectivism. Sociable atheists are less common than loner atheists in my experience. So look for solitary lifestyles. High solitude, self-imposed, reclusiveness. Hermit, but not monk (monasteries are too collectivist).

There is one important exception. William James, in "The Varieties of Religious Experience" defined (private) religion as your thoughts/behaviors you practice when you are alone.

This is "existential religiosity" as opposed to "organized religion." There are actually people who live by that. They are almost indistinguishable from existentialist atheists. Theirs is a private, screaming relationship with their god.

For the record, "existential atheist" defines me pretty well, and yes, I exhibit my conjectured key-indicator symptom of self-imposed isolation. I am not an extreme recluse of the J. D. Salinger type though... I just have low social energy (I have about 3-4 good social energy hours in a week; more than that and I feel like killing myself).

You want to cite Camus and Sartre more than Kierkegaard btw, since the latter eventually decided to go Christian because he decided it was the "most absurd" belief system. I am oversimplifying but that's the gist of it...

And yes, existentialism is among the most common self-rediscovered philosophies. Every smart introvert rediscovers it before encountering Sartre &co.

Venkat