Stock vs flow variables

How could I decide from a variable exactly, that is stock or flow type?

Variables, which are measured at a point of time are called stock variables whereas variables measured over a period of time are flow variables. This means that stocks are non-recurring in nature whereas flows are recurring in nature.

I found, that number of unemployed people is stock and amount of salary is flow. As I know, salary is a fixed amount of money, but number of unemployed people can be changed really fast.