The easiest way is to but an extremely low impedence, high power resistor inline with the device whose current you are measuring. Then use an ADC to measure differential - using the supply power to the resistor as the ADC voltage reference and the output from the resistor as the ADC input.
The problem here is that you have to balance power loss and accuracy. The very nature of the measurement is that you are creating a voltage loss which will vary dependent upon current. The lower your voltage loss, the less accurate your readings will be. A higher resolution ADC will help this somewhat.
In an application I am using a .01 ohm 1W resistor to handle a device which will draw <100mA @ 5v when it is maxed out, but will burst to 200 if the device is jammed. In this app, since I only have to tell the difference between 100mA and 200mA, I don't need a whole lot of accuracy, so this works fine.

Actually, In your app, this method probably will not work. You'll probably want to use an HES method; I don't think it would be easy to find an affordable low-impedence resistor capable of up to 50w!