It's a good habit to check the conversion rates before purchasing anything in foreign currency.
A quick way of doing this is to type the currency you wish to compare directly to Google.

But the results from Google are humble rates. If you are using a credit card, most likely, you are going to be charged by your bank's own rate.
To get the most accurate conversion, the easiest way is to check the rates with your bank online.

Here's how you do it in Citibank: