It is common in a simulation to add probes are used to measure outputs at a specific point in the circuit.
It is common in a simulation to probe voltage at multiple points in the circuit. You can add voltage probes to your schematic to measure the voltage at a specific point in the circuit.
If you also want one or more current probes, add AMMETER components from the ngspice-simulation library and place them inline in the circuit.
These probes are used in an AC Sweep analysis, to measure voltage phase relative to an AC source in the circuit. The sweep phase shift is delayed by reactive components in the circuit.