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But there's no way to test/verify/falsify that the rate of decay is constant for all spans of time and all conditions and the only way this could be verified is with a sample whose date is known. YOu are in the position of simply trusting that it holds up, but you can't know that.
No, that's not true. There are a number of steps physicists can take (and have taken) to determine whether radioactive decay rates change. First, you can understand why decay takes place, i.e. develop a good nuclear theory, and then predict based on the theory how decay rates would change for various conditions. Physicists have done that. Based on their theories, they predict that most decay rates would change not at all in any kind of environment relevant to dating rocks (e.g. we don't have to ask what would happen to decay rates at the center of a supernova or in a neutron star -- the fact that the rocks are sitting in an intact Earth sets constraints on what conditions they might have experienced). One kind of decay (electron capture) can change slightly as pressure increases, but this would involve no more than a small correction factor, and does not apply to most dating techniques.
Second, they can test their predictions: measure decay rates under increasing pressure, for example, or at different temperatures. The predictions turn out to be quite accurate.
Third, they can test lots of other predictions made based on the same nuclear theory. The same theory predicts, for example, how stars behave, and how nuclei interact when you slam them together in accelerators. Again, the theory appears to be quite sound. (Lots of details can't be calculated from existing theory; that's why physicists do experiments. But none of the uncertainty, and none of the experimental results, has given any hint that the basic conclusions about the constancy of decay rates are wrong.)
Fourth, they can use astronomy to see if nuclear reactions behaved differently in the past. Looking at stars that are different distances away, for example, shows that nuclear reactions (which power stars) behaved identically over a vast range of times, from a few years to many millions of years. (This is because light takes a long time to reach us from distant stars, so the light we're seeing now was produced millions or billions of years ago.)
Fifth, they can compare different dating methods for the same samples. It's hard to imagine any mechanism for decay rates to change, but it's much harder to imagine a mechanism that would have only the effect of speeding up all known decay rates (and without introducing any new unstable isotopes). Different dating techniques give consistent answers in the vast majority of cases.
Sixth, they can look at specific events that occurred in the past. Specifically, they can study the Oklo reactor, which was a natural fission reactor that occurred in uranium ore deposits in what is now Gabon a couple of billion years ago (according to conventional dating, that is). The particular mix of reactor products produced turns out to be a very sensitive test of whether nuclear reactions have changed at all since the reactor operated, and the conclusion is that they haven't.
You see, physicists don't just assume that physics, and decay rates, haven't changed. They have investigated the possibility of change as an interesting scientific hypothesis, and have used a variety of techniques to set very stringent limits on how much change can have occurred. There's an entire little subfield of physics that tests things like this.
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