Grenadilla and other woods used for clarinet bodies are dense and prone to cracking when exposed to rapid temperature or humidity changes, particularly when a cold instrument is warmed quickly by breath without a gradual warm-up. Cracks most commonly appear near the tone holes and at the top of the upper joint, where wood stress concentrates most; run your finger along these areas and inspect under strong light for hairline separations. A crack that reaches through to the bore (the inner tube) affects tone and airtightness and needs a professional repair (typically pinning) to stop it from spreading, while a very superficial surface crack may be cosmetic only. Given how common this issue is on wood clarinets, this check is arguably the single most important one for this instrument.