Welcome to the forum.
The reactors are designed to resonate with the capacitors at a frequency which is not near a harmonic frequency.
The reactors are typically wound on a three phase core, so mutual reactance is an issue. Additionally, the reactor is wound on an iron core, possibly with an air gap, but the inductance of the core changes with flux density. The important issue is the reactance at the rated current rather than the reactance with no flux.
The easiest way to test the reactor, is to connect the correct in series across a clean supply, and measure the voltage drop across the reactor. This will then give you an indication of the reactance at the full design flux.
To test a capacitor, connect across the supply and measure the current. Provided the harmonic voltage is very low, the current will indicate what the reactance is.
My reccommended filed test, is just current measurement.
When an installation is made, my recommendation is to measure and record the current drawn by each capacitor. Compare the current with the correct current for that capacitor. If the current is high, there is probably a harmonic voltage issue that needs to be addressed. If the current is low, then the capacitor has probably been damaged and lost capacitance.
Regular current checks are recommended to ensure the health of the capacitors.