Well in your games list instead of 3 dots it will have a pause symbol and if you enter the game it will say underneath 'your opponent is on vacation'.
"mozas is currently on vacation!
They have 8 Days 23 hours of vacation time left, but may be back sooner."
How can you tell if your opponent is on vacation? Just the fact that the clock is at zero and the game hasn't ended? And therefore, you have no way of knowing if they've already gone on vacation without waiting for the clock to expire?