If the drains are configured as sweepleader proposes above (vent tube at the high end, then side drains, with exhaust port at the low end), then it appears that there may be some Q&D tests which could've been done 50 years ago, and every year since, and might still be applicable today.
A simple pressure test: you plug the downstream exhaust port, and pour water into the upstream vent tube. For a new healthy drain, a known (calculable) volume of water would quickly fill the pipes, then a larger volume of water would more slowly fill in the under-slab gravel filters. If only a little water goes in, this indicates the pipes are clogged or cracked. If too much water goes in, this indicates voids have formed adjacent the drainpipes.
If this pressure head is held long enough, water should perk back up into the slab, with the wet spots indicating potentially previously unseen flow pathways. This could also be used to validate the efficiency of slab water sealing methods (which to date apparently haven't been too effective).
For drains that can not hold a pressure head, various tracer chemical salts could be added to the water to determine what percentage is migrating into adjacent drains, and therefore how much is exiting via unknown pathways. With the right tracers, it may even be possible to determine where the missing water reenters the river. A “map” of the underflow pathways could be quite useful.
When the sidewall plug is pulled out, the exit water could be evaluated for turbidity. Any suspended solids in the water indicates that much less backfill under the slab.
A key question for the State is: do they simply seal the upper spillway, reinforce it with more concrete, or rip it out and replace it. Having a better understanding of these subsurface drains aids in that assessment.
