One approach to radioactive dating is to measure the amount of C14 as well as the quantity of the nuclear decay products. The current quantity of the radioisotope + the decay products tells you the original quantity of the radioisotope. Once you know the original and the current quantities, you can figure out how many half lives have elapsed.
Much C14 dating is done by comparing the ratio of C14 to C12. That also gives a good idea of the elapsed time since the organism died.
C14 is constantly being created through interactions of cosmic rays with the atmosphere, and the quantity of C14 ingested by living organisms is proportional to their size. Once they die, they no longer ingest C14. The rate of formation of C14 is almost constant. It probably varies slightly with the activity of the sun, but since the suns activity also cycles, the variations are evened out over time.
The Khan on-line academy has a couple of lessons on C14 dating. Once you do the math yourself, you get a much better understanding of the methodology.
The problem there is that the vast majority of C-14 is generated by the solar wind and cosmic rays converting Nitrogen (N-14) into C-14, which then decays back into N-14. There is no way to accurately measure the proportions using the decay products. That's why they measure the ratio of C-14 to C-12 and calculate on the assumption of relatively constant creation of C-14 (an assumption we already know to be invalid, but not, we think, hopelessly flawed). So, even though there are dramatic fluctuations in the rate of C-14 creation, such as the spike in the late 8th century where, IIRC, about double the normal amount of C-14 was present (calculated by analyzing the individual rings of trees that were preserved by submersion in an ice lake). So, not only is there significant variation in the levels of C-14 present historically, we don't even have a complete record of that (yet - it is a work in progress). C-14 dating is no better than a best guess based on reasonably solid, if incomplete, data.