How do you quantify technical debt?

It doesn't seem possible to track percentage of code as debt. You can't put a comment on code and call this block either debt or good.

One way could be to measure different activities. If you spend time solving a lot of bugs (muda) then where are those bugs?

If they're in the same old code, does that code need rewritten? If they're in new code, is that code being written in a good way? Can better testing catch these?

"Technical debt" seems as hard to measure as developer productivity. It seems you need something you can actually put on a bar chart to measure the effects of having technical debt.