Left 木 is the indexing tree/wood radical - a tree in profile with trunk, branches, and roots. Anchors 检 because the original sense was 'to seal a wooden document tablet': bamboo slips bound and tagged with a sealing strip. From sealing came 'check, examine, inspect'. Same radical family: 林 forest, 松 pine, 树 tree.
Right 佥 supplies the sound - qiān drifting to jiǎn through palatalization, the same shift seen in 险 xiǎn dangerous, 剑 jiàn sword, 检 jiǎn check, 验 yàn test. The whole 佥-phonetic family clusters around the qiān/jiǎn/yàn rhymes. 佥 itself was 'all together' in classical use; here the meaning is irrelevant, only the sound.
