The best answer is you dont untill it is time to settle the bill. This is a big problem for third world country hotels that book in USD and then convert to local currency. They seem to make up a rate. I have pressed this issue at several hotels in the past. It has always lead to a better rate. I once got 5 different answers with a lot of BS. I had all the possible data and nothing matched. The poor gal at check out was just doing her job and I told her so but I kind of laid into the Manager when he was called. He was a bit of a dick. In the end it saved me about 6% or $100. I then took the $100 and spread it out among different staff before I left.