Mostly by looking at how long simple changes (e.g. changing a fee + fee message at checkout) that should only take a day or less, actually take.

But how do you know how long a change “should” take? And how do you know if the rewritten code would be any easier to change?

I tend to be sceptical of “rewrite everyting” projects since they are often based on wishful thinking.

Alice has arthritis, so it takes her 3min longer to write any functional changes. She, however, has domain experience across the entire application. Bob can write code 2min quicker than Alice, but is relegated to a specific section of the codebase.