honest q: how do you distinguish between single node erlang applications vs clustered erlang applications?

This returns true if you're on a single node erlang application :P

    erlang:node() == nonode@nohost andalso erlang:nodes(known) == [nonode@nohost]

A single node erlang application would be one that doesn't use dist at all. Although, if it includes anything gen_event or similar, and it happens to be dist connected, unless it specifically checks, it will happily reply to remote Erlang processes.

yeah, isolated + distributed processes is THE THING about Erlang, OTP

my personal reality is that the majority of projects I've consulted on have seldom actually leveraged distributed erlang for anything. the concurrency part yes, clustering for the sake of availability or spreading load yes, but actually doing anything more complex than that has been the exception! ymmv tho!

I mean you can certainly run one off scripts in elixir and Erlang.