The short answer
Good basmati has long, slender, uniform grains, very few broken pieces, a natural nutty aroma and a translucent (not chalky) look. It should be aged, and it should elongate and stay separate when cooked.
Quick checklist
- Length: long and slender, all roughly the same size.
- Broken grains: the fewer the better. Broken pieces turn mushy.
- Look: translucent, not chalky white.
- Aroma: natural and nutty. Strong perfume can mean added fragrance.
- Mixing: no shorter or cheaper grains blended in.
- Age: aged rice cooks fluffier.
The cook test
Cook a small cup. Good basmati should roughly double in length, stay separate, and smell even better cooked than raw.