how would you say you worked hard today in korean?
is it 오늘도 수고했어요?

Question

Answer

수고했어 is informal for like children or to your subordinates
수고했어요 is in between for like fellow coworkers
수고했습니다 is formal for everyone else not listed above

These are more used. Make sure you elongate the last pronunciation for polite engagement with people you say it to.

Answer

(Formal also talking to elders)
오늘도 수고하셨습니다.
오늘도 고생하셨습니다. 
오늘 하루 고생하셨습니다.
오늘 하루 수고하셨습니다.

(Formal)
오늘 고생하셨어요.
오늘 고생하셨네요.
오늘 수고하셨어요.