A long-span slur functioning as a phrase mark should be distinguishable from a short articulation slur

A short 2-4 note slur usually indicates a specific articulation/bowing/tonguing grouping (drawn close to the noteheads); a slur spanning many notes or multiple measures usually indicates an overall PHRASE MARK, conventionally a shallower, longer curve.

Gould distinguishes phrase marks from articulation slurs primarily by function and typically by curve shape; Read similarly separates the two usages even though both use the identical glyph.

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