The tracking method Beverly came up with only works on Changelings that were part of Project Proteus. Ten Changelings.Something hardly talked about so far:
The crew just learned a reliable way of tracking and exposing the infiltrators!
What are the chances this will lead to a nice montage scene of all changelings Starfleet wide being ousted and exposed like Hydra agents in The Winter Soldier?
Unless they make some excuse about 'contact with Proteus subjects infects other Changelings' or something. Which I guess could work, as Vadic can pass on her new ability, so maybe that downside is passed on too.