You will likely need to measure the amps drawn under stall, not just under "flat out" conditions. At least this is what my Digitrax decoders call for. You need to run the loco as David describes, but push down so the wheels (momentarily!) stop turning. This is when the loco draws the most power, and is what will fry the decoder. It is this value that will help you decide if the decoder is appropriate or not.
Andrew
I have to disagree with this approach, especially in light of the relative prices of locomotives and decoders these days.
Disclaimer: The following does not apply to locomotives with traction tires; these will usually stall before slipping their drivers. Stall current would be the preferred rating when a locomotive has traction tires.
None of the electric motors currently in use will survive very long being stalled with power on. The stall current is too high; the motors overheat, short out, and burn up (it generally takes a few seconds for this to happen).
The only exceptions are the stall motors used on Tortoise and similar turnout motors. These motors are designed to be safe stalled with power on. The downside of the stall motors is insufficient torque to power our locomotives.
Almost all locomotives (again, assumes no traction tires) in the smaller scales will slip their driving wheels before the motor reaches its maximum continuous current. FWIW, this is the way to safely weight your locomotive for maximum traction. Add weight until the wheels start slipping at the maximum continuous current rating of the motor.
You can then safely use the motor current with drivers slipping as a basis for choosing a current rating for the DCC decoder. After all, you are never going to see a stall current in real use on a layout.
Another upside: Should you somehow manage to get your locomotive locked up (wheels can't turn) with the power on, if the decoder is rated less than the stall current, you will likely fry the decoder instead of the motor. Since new decoders are easier to buy, cheaper, and easier to install than a new motor, I would rather fry the decoder than the motor.
But I've yet to see a derailment that locked up the drive wheels, but kept the power flowing. And you don't have to stall the motor, and chance it's overheating, to find the slip current. Simply block the locomotive from moving horizontally, and measure the current when the wheels slip.
For these reasons, I prefer the slip current for rating my decoders. I suspect the Digitrax advice was written back in the days when decoders cost more than locomotives.
my thoughts, your choices