A quick and dirty method is to read the voltage and current on the DC adapter. That will give you an idea of the upper limit. For example, an old D-Link router I have has a 5V, 1A DC adapter. So the maximum power the device will use is 5W (P =V * I). Really, there is also usually a 30% to 50% safety margin, so it's probably more like 3W in this case.
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A quick and dirty method is to read the voltage and current on the DC adapter. That will give you an idea of the upper limit. For example, an old D-Link router I have has a 5V, 1A DC adapter. So the maximum power the device will use is 5W (P =V * I). Really, there is also usually a 30% to 50% safety margin, so it's probably more like 3W in this case.