My personal way to do this is to check on the web what the rate without any fee is (search engine “100/however much you need eur to isk“) and then I make a mental note of that number. I go around exchange places and ask how much isk they’d give me for 100 eur all included. Makes it easier to compare them with each other without having to figure out whatever commission and fee structure they came up with, and gives me an idea of how far they are from the interbank rate.

An even better solution is to get a card like TransferWise (revolut are exploitative assholes and I don’t recommend enabling this employee exploiter) that charges very very reasonable rates so that you don’t even need to exchange or can use an atm if cash is required.