One issue with software methodology assessment is the difficulty in getting consensus on ways to determine code quality. One potential experiment I've been thinking of to work around this is by seeing if 3rd parties can tell if a methodology was used. To abuse OP's analogy of debug statements, this is essentially a "assert this practice makes a visible difference".

Have Alice and Bob sporadically use practice X on a group of changesets and track the time spent on each. Eve and other coworkers then go through the resulting list and try to determine which changesets used practice X.

There will need to be practice-specific variations of course. For example if experimenting for TDD, the changesets obviously need to include tests and code in the same commit (without indicating which came first). If experimenting for pairing, should probably report the time elapsed as the pair worked on it, instead of summing the time spent by both developers.

Just thought I'd throw this idea out here where it's somewhat on-topic and see what y'all think about its feasibility. If anyone tries this out I'd love to hear the results!