Just measure it with a multimeter between nodes 1 and 2. Or apply a voltage between nodes 1 and 2, measure the current flowing and then calculate the resistance using Ohm's law.
The circuit is in a box so can not access it. Also the ground is fixed can not apply voltage between n1 and n2....??
The losses due to the leakage resistance of the capacitor will be negligible. If you want you can put a resistor in series with node 1 to avoid an inrush current into the capacitor when you first apply the DC voltage. However, this will complicate calculations a little bit.