Rookie2:
I guess so, I could also use an Amperemeter connected in series with the +5v line of the USB cable, right?
Current over the USB interface is not likely to be the limiting factor, and in any case by the time you've measured the actual current it's a bit late to be thinking about whether you've just overloaded something, isn't it?
You need to read and understand the electrical spec of whatever it is that you're planning to connect, to ensure that it will not damage or be damaged by the Arduino.