"Unexpected token <"

[YCMitch]

So, just like everyone else, I sporadically get the error:
Unexpected token < in JSON at position 0 when trying to compile a Gatsby site.

I understand the usual response is "there's an error somewhere in your code" as the site is returning HTML instead of JSON.

Is there any way at all to actually read what's being returned? Nearly all responses say "just find out what's wrong", but surely there's a means of reading the error somewhere, rather than digging around in the dark until something works?
Seeing how common this error is, I'd assume there'd be some sort of reference to it somewhere.