I'm using vim with the detectindent script (with ugly hacks and enhancements)[1] for that, it gets this right most of the time if the source file isn't too short, including the number of spaces in case it's space-indented.

[1] https://termbin.com/qzwzy

Personally, I'm a tab user in a language that does mostly spaces (Rust). My own projects use tabs but most projects I interact with use spaces. I had tabs in my editor for a long time and I would always either press the space button or change the editor's setting. Then I got sick of it and changed the editor's default. My own projects have an editorconfig file that my editor can read, setting it to tabs in the project's scope. The projects I contribute to usually don't have such a file, that's where the spaces-per-default setting comes in.

Why not use editorconfig? https://editorconfig.org/

right, and I would expect most editors to indent automatically these days. Do people really routinely indent code by hand?