The top 4 tone-holes are the most likely ones to have humidity cracks. .
These are the most common types of cracks. When you play, you have a moist climate inside the instrument of 100% humidity and a temperature around 80 F. The humidity will enter the wood at the tenons, sockets, tone holes, and the tapered part of the bore. Since clarinets are poly-cylindrical or tapered from the barrel tenon down to the G hole, this is the area where almost all cracks occur. Water will enter the tone-hole tenon and bore section and make the wood swell from the inside. The water will only penetrate the wood a few millimeters deep but humidified wood increases in size considerably and will expand. This creates enormous tension. The outside wood on the contrary is dry. The pressing inner wood will finally cause the wood to crack. If the cracks are not touching any tone-hole they are just surface cracks, going only a few millimeters deep and have no acoustical effect. They are just optical irritations and need to be glued, cleaned and polished. Dont oil or grease them. Just put a piece of tape over them and get it fixed when the time allows. If a tone-hole in affected, the instrument will leak and becomes very unreliable. Check out the the article about tone-hole cracks.
Be aware that some of these cracks are hard to detect, because they are placed just underneath the top side trill keys.
Dont get the instrument pinned under any circumstances!!! (Read also: Crack pinning )