even just by looking at만 means “only” or “just,” and 봐도 means “even if one looks” or “even by looking.” Together, 만 봐도 commonly means that something is clear from merely looking at it.
KO: 표정만 봐도 그가 얼마나 피곤한지 알 수 있어요.
EN: You can tell how tired he is just by looking at his expression.