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Magnetars have gained favour as the engines of extreme supernova openParSNclosePar explosions comma but direct observational evidence is lacking. A unique characteristic of a magnetar engine is non-thermal X-ray emission a few years after the explosion. This has so far eluded unambiguous detection due to the extremely high absorption by the ejecta itself before it expands sufficiently comma and the rapid decay of the magnetar luminosity. To balance this trade-off comma we propose to observe a small sample of SNe at intermediate epochs when the ejecta are expected to become transparent to soft X-rays. This allows us to detect or constrain the properties of the magnetar and shed light onto the engines of these extreme SNe.