Consider a light with two duplex switches. The switches work as I can turn light on/off from either switch. However, I'm not getting any voltage reading on my meter. The meter does work as I can get a reading on another circuit. This is existing wiring and I don't know the wiring plan. I want to know where the fixture is in relation, wiring-wise, to the switches (before, between, after). Where should I place my meter leads to correctly measure voltage in duplex switches?
What you are looking for is called "Three way switches", although it will only be two switch locations. There are several wiring schemes that can be used, depending upon where the main power is derived from, switch box or light fixture. This link explains 8 different scenarios.
One box will have the feed voltage, 120 volts ac. It can be in either of the switch boxes or the light fixture box. Any readings across the switch terminals would depend on the switch settings and that would require a light bulb installed that would complete the circuit.
Because the switches only control the hot leg, you'll have to measure between ground or neutral (if it's available). Three-way switches will have 4 terminals, 1 Common, 2 Travelers, and 1 Ground. The ground terminal should be green, or on the top or bottom of the switch. There should be 2 screws of the same color (brass usually), and one off color screw (black). The off color screw is Common, and the 2 similar screws are be travelers.
For 3-way switches to work, the hot from the breaker will connect to the Common terminal on one switch. The fixture(s) (light) will be connected to the Common on the second switch. The switches will then control which traveler is hot, so the travelers will be hot or not depending on the switches positions.
If you are only trying to determine which switch is wired to power, and which is wired to the fixture(s). The safest way is to use a non-contact voltage tester. With the power on, flip the switches so the lights are off. Bring the non-contact voltage tester near the Common terminal on one of the switches, if the alarm goes off you have found your feed wire. If not, test the other switch in the same way.
If you are determined to stick metal probes in a hot circuit, you'll first want to locate a solid ground. Place one probe on ground (not the ground... Electrical ground), then Carefully touch the other probe to whichever terminal you want to check for power.
If you don't feel comfortable with any of this, don't hesitate to contact an Electrician.