Firstly, just make use of the Internet. Google is a fantastic online search engine which is able to offer you some instant answers. As well as its 'define:' function, Google offers calculator and instant translation services when it comes to cash. Let's say you are trying to convert 10 UK pounds into US dollars. You would simply need type '10 pounds in dollars' into Google, and you should be provided with an instant answer under the search bar, just above all of the website results. This number will be based on the latest exchange rate number that has been found and hence for the time being you can rest assured that this is an accurate reading.
If you don't trust this, then there are places you can visit to find out. There are many bureau de changes located across the world, and many of them are found in travel stores. Simply go to your nearest mall or shopping area, and there should be a store that will be able to help you. Not only will they be able to sell you foreign currency but they will also be able to tell you the exchange rate that they are working from. This will give you time to find out which one is offering the best rate, and a chance to get your money converted whilst you are there.