zhì is a narrow reading meaning 'to mark, to record'. It is alive in 标识 (a sign or marker) and 标识符 (identifier). Note that 标识 is often pronounced biāoshí in casual speech, but the dictionary reading for this 'mark' sense is zhì. Everywhere meaning 'to know' the character is read shí.
The speech radical, a left-side simplified form of 言. It places 识 in the language family — in classical thought, knowing is bound up with naming, so characters with this radical cluster around speech and identification, like 说 and 认.
只 supplies the sound in the simplified form (traditional 識 used a fuller phonetic). After the reform 只 was chosen as a graphic shortcut; the sound shifted from zhǐ to shí, the central reading of the character.
