There is no independent frame of reference by which we can tell whose time is slower.
From the spacecraft's frame of reference, Earth's time is slower. From Earth's frame of reference, the spacecraft's time is slower. Both are right.
When we say that time is slower or faster on a spacecraft, the Moon, or an exoplanet of Christopher Nolan's, we are implicitly prefixing the statement with "From Earth's frame of reference..."
From the spacecraft's frame of reference, Earth's time is slower. From Earth's frame of reference, the spacecraft's time is slower. Both are right.
When we say that time is slower or faster on a spacecraft, the Moon, or an exoplanet of Christopher Nolan's, we are implicitly prefixing the statement with "From Earth's frame of reference..."