I've read a lot of question about testing, so this may be a duplicate, but so far, I've been unable to find this question.
Following WW3, and its scenarium-radiating super-weapons, mutants appeared in our societies. In fact, newly super-powered humans were crucial in ending the war, or at least putting it in an indefinitely long cold war state. (You have a nuclear warhead? That's cute, we have a woman here that can safely take ten of those apart mid-flight without breaking a sweat).
Mutants resources are now a central element of geopolitics, war strategy and culture in general.
Fast forward a few dozens years after the agreement on a cease-fire. Mutants are statistically less powerful and the few Tier-1 are catered and nursed by the government, juuuuust in case.
The less powerful ones (aka, those who won't destroy a city should they sneeze) are allowed to take on super-hero duties and help keep the super-criminality in check.
Of course, the government registers everybody (mutants and non mutants) for obvious reasons. You have the right not to use your powers (unless you're some kind of superhuman shield missile, in which case you'll be kindly escorted by a platoon of mutants powerful enough to bash your head in and voluntarily enlisted). But you have to register.
Method of registration
Mutations reveal themselves thanks to the heavy hormonal unbalancing associated with puberty. Upon reaching majority, a mandatory registration is done. During registration, each subject is prompted to list all of their abilities (to the best extent of their knowledge) and demonstrate it in a safe and controlled environment. (Demonstration help keeps a power rating, used for various things).
Should you be found guilty of perjury, the consequences would be really unpleasant (including, but not restricted to: prison, "communal work", mandatory military service, life-long imprisonment, exile, death-sentence, life-long slavery).
For many reasons, mutants could and will try to conceal some of their abilities - and my MC is certainly going to try to.
Other than the screening and incentive listed above, what ways my totally not totalitarian government might use to ensure the best knowledge possible of all mutants respective abilities?
No, my Cold War government can't hand baskets of fruits, or any other material niceties to all mutants. We're still at war and the tax-payers are already funding the wall. (But if you want to add reasonable measures, such as a quota of mutant in companies, that can be arranged)
With a slightly more advanced technological level - 50 years or so - you won't be able to hide your abilities if you use them. Someone is going to take a video and post it on social media, your neighbour will cough to the authority and so on.
The mutant population rate is a little less than 0.1%, and top interest mutants are 0.01% of this population.