@Zexi Liu if you have a voltage source

Vfoo

within a subcircuit

X1

the voltage source current is

i(v.x1.vfoo)

. If

foo

is a

X1

internal node, the voltage on foo is

v(x1.vfoo)

or simply

x1.foo

. The ngspice

display

command after a simulation will show you all saved node names.

Thank you!