This discussion, as always, gets very muddled because posters use different terms with different means that are not always clear. There are three parts to an ATM transaction -- the ATM itself, the network, and the card issuer.
Whoever owns the ATM gets to set any fees for just using that ATM. In some fifteen years of using ATMs in Europe I have never seen an ATM charged a fee for using the ATM. However, in the US that is common when you use an ATM not owned by your bank. The ATM does NOT set the exchange rate, it is just processing your card.
The next level is the network that handles your card - Cirrus, Plus are the most common. There some other minor networks but I would make certain that my card works on either. Credit Union in the use tend to also use STAR. The network sets the exchange rate - that will always be the Interbank rate - PLUS a fee of about .75% for handling the transaction. That fee is buried in the exchange rate and cannot be avoided.
The network then presents that charge/withdraw to your card issuer/bank. This is where it gets messy and confusing. The bank can add anything it wants to the withdraw - withdraw fee of $5/transaction, a 1 to 4% currency handling fee, anything - BUT those fees must be fully disclosed (class action suit 20 years ago). This varies greatly and only the card issuer can tell you the fees. i.e. - my credit union charges 1% on the first six transactions in a 30 day period. After that it is 1% plus $1 per transaction. My back up card issued by my bank is $5/transaction and 3%.
Even if you card issuer charges fees, it will always be cheaper and more convenient to obtain local currency via a bank owned ATM.
PS - Knowing the exact Interbank rate is very difficult because it is a fluid rate that literally changes every minute. Most of the rates that you see published in papers are an average for the day or close of market or some single point in time. Even exchange rates on the internet will lag the real rate just because of time delay. But the difference will be in the three and fourth number - .00X or .000X. Short of a major financial crises the exchange rates rarely move more than a penny or so each day.