Considering a rewire in the future and want to figure out which outlets (and ideally fixtures) are knob and tube. I know many are, some are not. Probably all 2 prong outlets are k&t, possible some 3 prongs are as well.
Is this best done with a multimeter, an outlet tester or something else? What are the steps then? Thanks!
There is no electrical or technical way to tell if a box is wired with K & T. You'd get the same results with old non-grounding NM cable.
The only way is visual. You'd have to open each box and check with your eyes.
The copper wire used in knob and tube has identical electrical behavior to copper wire in any modern cable assembly. Determining the wire type (sheathing around the copper) and cable assembly or use of conduit is something that can only be done visually.
Opening up the electrical device boxes (make sure you understand how to work safely on electrical equipment if you proceed with these steps) and inspecting the wire coming into the box will allow you to determine the type of wire feeding into the box. Also, tracing visible portions of wiring in an open attic or basement also provides good clues. That said, if any work was done on your home without a permit prior to your ownership, it is certainly possible that the work was done by a homeowner or handy man unfamiliar with proper electrical code and safety. In these cases, it is not uncommon to see modern wiring in a device box with an illegal and unsafe splice/junction concealed inside of the wall.
Regarding your guess that 2 prong outlets are knob and tube while 3 promo outlets may be modern wiring is a good guess. That said, the three prong outlets may still be knob and tube even if an outlet tester reports proper grounding because it is permissible to run separate ground wires to ground old wire in some situations, so that still brings you back to visual inspection as the only way to confirm the wire type.
I don't know of a tester that'll tell you about wiring type. Your best hope may actually be looking at how wires enter the junction boxes, as well as a borescope in any openings therein. Borescopes are quite inexpensive of late and attach to a phone as their power and display. I would use a plastic-bodied one and turn off the power.
What people worry about with K&T is wiring failure. This is easily detected by an arc-fault breaker (AFCI). The AFCI will have trouble if several K&T circuits share a neutral, but that was never a common practice for the same reason it's not today: 2 hots sharing a neutral overloads the neutral unless it's set up just right (MWBC).
There was another worry which was debunked. Consider Romex, run through walls packed with insulation. Despite both conductors packed tight together in an insulating sheath, they don't have any trouble cooling. For some reason, somebody thought Knob-n-Tube would have a problem with that, particularly blown-in insulation done as a retrofit. Further research proved that to be false: insulation-packed K&T didn't have any worse trouble than otherwise. So states have started allowing blown-insulation over K&T.
The third issue with Knob-n-Tube is no ground. NEC 2014 liberalized the rules for retrofitting grounds, so you can add grounds wherever needed.
If it was me and I suspected K&T, I would install AFCI breakers, and retrofit grounds as needed. The cost of a big wiring tear-out is better spent on other safety concerns.
