An anacrusis's beat-deficit must be exactly compensated by the final bar of the piece (or repeated section)

If a piece opens with an anacrusis D beats short of a full bar, the final bar of the piece must be exactly D beats short as well, so pickup-bar + final-bar sum to precisely one full bar.

A piece with a one-beat 4/4 anacrusis (opening bar 1 beat, a 3-beat deficit relative to a full bar) whose last bar is a full, uncompensated 4 beats.

Gould, Behind Bars, ch. 'Bar lines and repeat marks'; Read, Music Notation, worked pickup + shortened final-bar examples.

This rule can be decided from pitches, durations and voices — no layout required. A validator can check it before anything is drawn.