I have used @xbarbie's solution for years and while it works perfectly for text files it does not work well with larger audio files on my raspberry pi. If you have hcs available and you want a different solution I have this abstraction that takes symbol input and outputs a 1 or 0 just like @xbarbie's. Relative paths work as well.

file-exists.pd

Oh my goodness thank you @whale-av for the contribution to such an old thread. I figured out how to integrate poly 1 1 as mentioned above and my solution looks like this:

Hopefully this and your patches help anyone who comes along in the future.

I'm personally still trying to figure this out. Does anyone know how to integrate poly 1 1 into this correctly for the note-offs?