The 270 Mbps is the theoretical maximum of the type of wireless network you have. One way to measure actual speed is to transfer a file from your computer to time capsule and measure the time it takes for the transfer to complete. With the internet things are a bit shadier. Your actual speed depends on a number of factors that cannot be counted as constants: the number of users on the exchange, your distance from it, the quality of your line, the quality of the cables used to connect you to the exchange. There are a number of web based speed checkers that will give you some speed measurement, which is usually inaccurate. One not so bad is http://speedtest.net. It uses the nearest server to where you are, which makes its measurements a bit more accurate. The best test would be to download a file from a server near your area but even this may produce inaccurate results depending, of course, on the work load of the server or the number of users in the exchange at the time of the test.
