How do we know the rate of decay for radiometric dating is constant?
First of all, you shouldn't take creationists seriously; like flat earthists, their views are totally out of touch with reality. Rates of radioactive decay have been tested many times in the laboratory and found to be accurate. However, there have been some instances where a substantial change of temperature or the formation of a chemical compound involving the radioactive isotope produces a small change in the rate of decay. When used for radiometric dating, this is not a source of error. The main source of error is the minuteness of the samples that labs have to work with. For example, only one potassium atom in 100 is potassium 40,the half life of which is I,300,000,000 years. In argon potassium dating the volcanic tuffs the lab has to analyse contain only a tiny amount of potassium 40,and only 11 percent of that decays to form argon 40 (the remainder forms calcium 40). The lab has to detect and measure the tiny trace of argon which has formed in the sample since it was emitted by a volcano a few million years ago.

Sometimes several dating methods are used on the same fossil or artefact, and if they are in rough agreement the dating is reliable. If you are looking for pin point accuracy, you won't get it with radiometric dating, but if accuracy to within a few percent is good enough, then that is usually achievable. The colossal orders-of-magnitude errors which creationists try to convince you of never happen.