The currency conversion is applied when Apple makes the payment to you, not when the customer makes the purchase from Apple.
You can see the rates used on the "Payments & Financial Reports" screen in AppStore Connect, after the payment has been made to you. (Before the payment is made to you, the screen shows an estimate.)
There is no API to download this, but you can download a CSV file by clicking on the small square with an arrow pointing down on that page.
I filed a bug requesting an API to download this (FB5446792) in 2017, which was closed as a duplicate. Feel free to submit your own!