We currently validate the multi-router is working via a self_test present under the "core" folder. 
Concurrency suites do not include the "core" folder and as a consequence they don't self-test the multi router.
We are concern the multi-router might not be running correctly for concurrency suites given no bugs has been discovered yet.
The self_test should just run a simple concurrency test and verify that 

• the db.getMongo().isMultiRouter is true
• if you run X number of operations, you obtain a distribution over at least 2 mongos
  Note X here should be big enough (like 100) so that we have a certain probability the distribution will happen and we won't pollute tests