Ian Macintoshon Stack Overflow • Recommends
On “How to debug a GLSL shader?”
You can try this: https://github.com/msqrt/shader-printf which is an implementation called appropriately "Simple printf functionality for GLSL."
You might also want to try ShaderToy, and maybe watch a video like this one (link to this video) from "The Art of Code" YouTube channel where you can see some of the techniques that work well for debugging and visualising. I can strongly recommend his channel as he writes some really good stuff and he also has a knack for presenting complex ideas in novel, highly engaging and and easy to digest formats (His Mandelbrot video is a superb example of exactly that : link to this video)
I hope nobody minds this late reply, but the question ranks high on Google searches for…