"By their fruits will you know them"
Wild mind controllers will grow in their powers and use them for their benefit, perhaps without even being aware of what they're doing. Lots of people stare at one another all the time, instinctively thinking things like "Come on, say yes" or "Sign the agreement, sign the agreement!" or "Tell me I passed!".
But for some of them, it works.
So, your 21st century government sets out to identify - using Big Data - cases of unexplainable / illegal behaviour on the part of clerks, bank directors, managers, and so on. People who routinely get on the bus without a ticket and aren't fined. People who are caught speeding or running a red light and are left alone.
Then, these people will naturally gravitate towards jobs and positions where their powers can work the best. Someone finding they're very good at selling anything will tend to become a salesperson, and so on.
Once you've got a suspect, they can be tested without revealing the secret by using some unaware person as bait. For example I could tell a cop to stop my candidate on pretext of speeding, and give them a ticket no matter what, even if they weren't speeding at all, and maybe search the trunk too, because Uncle Sam needs that car to remain motionless on the road for fifteen minutes - don't ask why. Just keep that car still. The cops stop the car and the driver knows not to have been speeding, so concludes the cops must be mistaken, and tries very convincingly to tell them off -- and succeeds. The cops come back empty-handed, tail between legs, mumbling they can't explain what took them. Gotcha!
Done cunningly, this could work even if the esper can read minds - just supply the cops with suitable false information about the reasons they're doing this stunt, that the esper might believe aren't related to mind-control powers.
Lots of other possibilities exist, depending on the mind control range and whether espers are themselves immune to mind control or not. Controlling espers might need to resort to anesthesia and surgical implant of trackers plus remote-controlled explosives on a deadman's switch.
In The Demolished Man by A. Bester, the "esper" test is conducted on aspirants in a waiting room by a lone esper transmitting to all of them "if you can hear me, go through the door with the ACCESS FORBIDDEN - STAFF ONLY sign". This case is in some ways the exact opposite. If espers are immune, one could supply some stimulus that everyone in a certain area would respond to - e.g. an evacuation order - and at the same time control everyone to make them stand still and look hopelessly around. The ones that do so will just believe they don't know what to do, the ones that don't are espers, even if they're unaware of the fact.