farseer wrote:
Hi,

Item, (1)
Is there a way to send a signal of any kind from the switch node to
the fixture and test if it gets there?
Suggestion; You clearly understand the idea of the first switch
'sending' the voltage to the second switch. Suggest that as a next step
you make sure it is 'arriving' at the second switch.
Depending in which position is the first switch the voltage will be on
one or the other wires to the second switch. OK so far?

Then depending on the position of the second switch the voltage will or
will not be sent to the light fixture/s. Once you know it's arrived at
the second switch you can figure why/how it's getting, or not getting
to the fixture. Yes there is a way; just rig up a temporary connection
to some source of AC voltage from either a live wire within that switch
box or from an extension cord. that should 'prove' the wiring to light
fixture.

Item (2)
Also, what is the proper way measure the voltage of light fixture.
this light fixture has a bx cable with white and black wire. The way i
have been testing using my multitester is touching the red probe to
the black wire, and using the fixture itself as the ground. Is that
correct?
Suggestion: Good enough, assuming all grounds are OK. You are only
looking for voltage. But many test meters are far too sensitive and can
sometimes give false/misleading readings.
Its often better to use a bulb, either in a holder with a couple of
short leads or even just touching one side of the bulb base to ground
or neutral and the other to the wire you are testing to see if it is
alive.
I used a bulb that way tonight to make sure voltage was arriving a new
switch for some workshop ceiling lights (just touched the base of the
bulb temporarily borrowed from nearby cos i was too lazy to go upstairs
and get a spare), to the wires. It was; the bulb lit. Clearing up later
couldn't figure why the stair light didn't work. I had borrowed that
bulb for testing cos it was easy to reach!