One way is to date things that you already know the age of. Libby did this when he first developed the method, by dating artefacts of Egyptian sites, which were already dated historically. Another way is to use tree rings. Every year a tree leaves a ring, the rings increase in number over time until a pattern of rings is formed. Sometimes the tree has many hundreds of rings.
Scientists can date the age of the tree by counting and measuring the rings.
Radiocarbon daters can then date the tree rings and compare the dates with the real age of the tree. This is a very good way of testing radiocarbon, and we now know that there are some differences in radiocarbon dates and real time. Most of the time radiocarbon dating is accurate, but sometimes it is different from the real age by a small amount. Using a calibration curve, which is based on radiocarbon dates of tree rings over the last 10000 years, radiocarbon daters can correct for this problem. We can also test radiocarbon by comparing the results with the dates produced by other dating methods, and there are many of those.