For a simple analogy (from what I understand at least ) Imagine it like this:
your dealing with a bridge. The voltage is rate at which the cars can cross the bridge (assuming uniform cars). There are far more cars that you can ever hope to cross on the other side of the bridge, so the bridge will always be "saturated" with the cars. Therefore it's easy to test to see how many cars can go through the bridge, as that the the number of cars coming out of the bridge.
However the amperage is like asking how much weight can I put in these cars?
How do you test that? Well, you just keep putting more weight on the cars until the bridge collapses, and then you build another one and post the limit ;P
Or, you keep adding weight and look for telltale signs of failure, and then back off. You can't just measure the weight at any given point of time to figure out what the maximum weight it can hold is.
(also imagine that the "Weight" that these cars are pulling across the bridge is the "power" rating of the circuit, in watts)
I think that makes sense. So basically just take the manuf's word for it