ALM - The way the 8400 series sensors work means that you can't just feed it some DC values - You need to simulate the sensor with the appropriate sampled AC signal etc. The sensor and the meter join together to create one "Hybrid OpAmp".
You could probably make one using the 11683A schematic and a dead 8481A though.
EDIT: I thought I might check to see if there was a way to measure the resistance as a quick check and the manual does have a procedure for when you don't have a known good power meter/sensor
Troubleshooting - Eliminating the Power Meter and Sensor Cable
Where a “known good” power meter and/or sensor cable is unavailable, another means must be used to isolate the fault to the Power Sensor.
This is done by ensuring the power meter is providing the correct 220Hz drive signal. Check the following levels of the square wave with an oscilloscope.
• At the black/white wire: -0.05 ± 0.05 Vdc (top of square wave).
• At the brown/white wire: -9Vdc (bottom of square wave).
If the levels are incorrect, then the power meter or sensor cable is at fault. Refer to the power meter service manual for troubleshooting information.
If the levels are correct then the Power Sensor is at fault. Continue by troubleshooting the A1 Bulkhead Assembly.
I'm not sure if this would rule out the corner cases but it seems to be a good starting point to at least identify if the sensor is good.
EDIT #2: As Stray Electron says if they aren't guaranteeing them as tested and you're not paying in the US$300-400 range then they are almost certainly dead. It's easy to blow the thermocouple in the units.
You actually might have better luck with an 8481D as these are diode-based and while they only range up to -20dBm they're fairly safe up to something like 20/30dBm (from memory, never taken one up there so no empirical evidence).
TonyG