There are three ways to find out the cross-sectional area. One is to measure the resistance of a known length of wire and then look up the size from a standard wire gauge chart. This is practicable only for very long or very thin wires.
The second method is to use a wire gauge tool. This has a number of holes or slots of different sizes. Find one hole/slot that matches the wire. Then look up the size as above.
The third method is to measure the diameter of a wire with a tool such as a pair of slide callipers and then calculate the cross-sectional area from the formula of a circle.
When using these methods, particularly the last two, to find out the cross-sectional area of a multi-strand wire, first find out the cross-sectional area of one strand and then multiply that with the number of strands in each cable.