The best way for all users would be if PayPal would display those fees before you make the transaction so that you can check them before sending the money. They do not display them however and a service like the PayPal Fee Calculator comes in handy to determine the fees that you have to pay.

You select the country and currency where the payment is coming from and send to, the monthly sales volume and some optional settings like 2.5% conversion fee, Credit Card fees or eChecks. A user can then calculate the fees if he would receive money or if he would send money to another user.

Once a fee is calculated you can take a look at the fee breakdown to see how the fee has been calculated. A currency converter is as well on the website to help you out with that as well.

via Donation Coder Forum