What have I marked as --assume-unchanged? Is there any way to find out what I've tucked away using that option?

I've dug through the .git/ directory and don't see anything that looks like what I'd expect, but it must be somewhere. I've forgotten what I marked this way a few weeks ago and now I need to document those details for future developers.

You can use git ls-files -v. If the character printed is lower-case, the file is marked assume-unchanged.

To print just the files that are unchanged use:

git ls-files -v | grep '^[[:lower:]]'

To embrace your lazy programmer, turn this into a git alias. Edit your .gitconfig file to add this snippet:

[alias]
    ignored = !git ls-files -v | grep "^[[:lower:]]"

Now typing git ignored will give you output like this:

h path/to/ignored.file
h another/ignored.file