I can't believe I've been going for years without koinwng that.

This is probably because your capacitor is defined in the Extract Definition file as a subcircuit device and not as a capacitor. In which case, it should extract out L&W as parameters of the subcircuit. These values are then passed down to the subcircuit which is defined in your model file. At which point, LVS or T-Spice, whichever one you are running will calculate the exact capacitance value as defined in the formula in the subcircuit definition. If you are not seeing the L&W values being extracted, then there is probably something wrong with the device definition in the Extract Definition File (.ext).

I am extracting a capacitor layout in v15 but I don't see the extracted capacitance value in the netlist. It just shows the instance of Xdevice like this:

X2 Cap_5x5

How do I see the capacitance value? Any ideas??