Germination rates for any given variety of seed are achieved by actually taking samples from specific lots and germinating them in a controlled environment. The rate literally refers to the number of out 100 (or more) seeds which germinate successfully within an acceptable period of time. These tests are normally conducted by government certified laborotories.
The Canadian Food Inspection Agency establishes minimum standards for germination rates of all vegetable, flower, and herb seeds for sale in Canada.
Generally speaking, the germination rate for nearly all seed will diminish over time as it is stored, so fresh seed generally has the highest rate of germination.