One can see how tired they are by the look in their eyes.
You can tell how tired a person is by looking at their eyes.
ご質問ありがとうございます。
① "You can see the tiredness in their eyes."＝「疲れの度合いが彼らの眼で見える。」
② "One can see how tired they are by the look in their eyes."＝「彼らがどれほど疲れているかは、彼らの眼の様子で分かる。」
③ "You can tell how tired a person is by looking at their eyes."＝「その人がどれほど疲れているかは、その人の眼を見れば分かる。」
ご参考までに。
アメリカ合衆国 
