How do you know the voltage drop of wire?

Voltage = Current x Resistance. To calculate the voltage drop of a piece of wire, you would have to know the current flowing in the circuit and the resistance of the wire. The resistance of wire depends on the material it is made of, the length, and the cross sectional area (also called gauge or AWG). Short thick wires have less resistance than long thin wires. You can look up the resistance of the wire on the Internet, and you can measure the current flowing by connecting an ammeter in series with the circuit. Multiply those two numbers and you will have closely approximated the voltage drop across the wire.