Right strength radical — pictograph of a flexed muscular arm or a plough. The indexing radical, marking
勘 as an action requiring effort: to investigate, survey, proofread — all tasks demanding focused exertion. Same radical groups
勘 with
助 (assist),
努 (strive),
劳 (labour),
加 (add). Effort applied gives the original meaning 'to scrutinize carefully'.