Problem:
- There is a mind control magic. Exact mechanism and details unknown and may vary from mages. What's known is just that they can control mind.
- Cannot tell populace that such magic exists.
- Government must find mind control magicians.
Solution:
- Establish an accurate method to identify mind control magicians (later called MCC)
How:
- Recruit at least one MCC
- Recruited MCC should be in charge of recruiting/detecting other mind controllers
- Try to narrow down who might be a 'wild' (unrecruited) MCC. In this step, manual labor and surveillance is required (e.g.: looking for not very talented people who achieve more than their ability as stated by other answers)
- Recruited MCC will be supplied list of person who might be a mind controller (narrowed down by other agency/branch)
- Recruited MCC will try to determine whether the 'suspected' MCC is really a MCC or not: recruited MCC will control the suspect MCC's mind to mind control someone. If it's a success then the suspected MCC is a positive (really a MCC). If the proxied mind control attempt is failed then the suspect is a negative.
- Since MCC details are not very clear, it's also possible that an MCC cannot be mind-controlled by other MCC. If so, then whenever recruited MCC encountered a target he/she cannot control, then the target is obviously an MCC.
Example:
- Alice is an MCC recruited by government.
- Bob is a suspected MCC (not known yet if he is actually an MCC or not)
- Charlie is a normal person who volunteered to be test subject, he waits at designated place (e.g.: public park or inside some office building)
- First, Alice try to control Bob's mind to go to where Charlie is.
- If the mind control is failed, then Alice knows immediately that Bob is definitely a MCC. She then calls another team to arrest/lobby/recruit/whatever they do with a MCC.
- If the mind control succeed, continue to next point
- Alice give command to Bob's mind that said: "control Charlie's mind to make Charlie stand up/sit down"
- If Charlie do as Alice's command, then Bob's mind control attempt succeed. At this point Alice knows that Bob is a MCC.
Other ideas:
- Make the existence of MCC an urban legend (e.g.: leak some information about it in occult site or occult TV program)
- Also make sure the urban legend is not taken seriously (e.g.: broadcast some comedy that make people who proclaim themselves MCCs are stupid, delusional people, and that existence of MCCs are just fiction made for entertainment)
- This will make real MCCs feel agitated: the leaked information is right, real MCCs feels that something is wrong (imagine you awoken an ability that you yourself don't understand very much, but everyone knows a lot of such ability and say that such ability is delusional)
- Example:
- There has been an established social norm that MCC is a fiction and people who claimed to be one is delusional clown.
- Bob awakened as a MCC.
- If at this point Bob make a big commotion about his ability, then the government can find him immediately
- If he did not, then continue
- Bob knows little about his own ability.
- Urban legend leaked information says that MCCs effective range is no longer than 10 meters.
- Not even Bob knows his own range, so he tried to verify this 'urban legend' by practicing his ability
- Bob finds out that those facts in government-spread urban legends are mostly (if not all) true
- If Bob tries to find further information about the urban legend (e.g.: via internet search) then government can try to locate/profile him (e.g.: government-built sites that contain the urban legend will track and try to profile its visitor from IP address, tracking cookies, etc.)
- If not then Bob will (hopefully) show sign of discomfort. Your agents might want to look for such people