A tuplet shows the numeral alone by default; the full ratio is shown only when the numeral alone would be ambiguous

Common tuplet types (triplet = 3 in the time of 2, quintuplet = 5 in the time of 4) are conventionally shown with just the numeral. The full colon-ratio (num:denom) is shown for less common ratios where the numeral alone doesn't unambiguously communicate the intended space, or in teaching contexts where the ratio itself is the point.

Gould, Behind Bars, ch. 'Tuplets', numeral-alone vs. full-ratio convention; Ross, The Art of Music Engraving, agrees for standard practice. This is also the confirmed, exact behavior of VexFlow's `Tuplet` class: `this.ratioed = Math.abs(this.notes_occupied - this.num_notes) > 1` when `options.ratioed` is unset — a standard triplet (num_notes=3, notes_occupied=2, diff=1) defaults to numeral-alone.

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