This is impossible for Monzo to show as the rate is decided by Mastercard, which only “settles” a few days after the transaction.
You can see here for previous exchange rates, but any general exchange rate graph will show you prior rates.
As for exchanging cash, you could use a service like Revolut or Starling to “lock in” your currency before travelling - This is good if you want to know exactly what you have spent, and not want to worry about changes in currency whilst away.