How do I know how much power is being used?

How do I know how much power is being used? How do I know how much power my PSU has left? I'm gonna put more fans on soon but I am just wondering if there is any way I can find out if my PSU has enough in it. Thx.

Here's some useful info;
http://www.howstuffworks.com/power-supply3.htm

Tiger said:

Here's some useful info;
http://www.howstuffworks.com/power-supply3.htm

Click to expand...

great link... makes me think i should of got a 550w psu lol..