Both are testable and both are observable. Humanity knew the Earth was round long before we were able to send one of us up there to see it. Following your logic, dinosaurs were stone skeletons because we have never seen one that wasn't (or have we?).
What test would you run to see how old the earth was?
Yes, humanity knew the earth was round long before space travel.
We have present evidence that we can observe for the existence of dinosaurs. Not only is that evidence bones, as you know, but also skin and organ imprints, though these are extremely rare, as even dinosaur remains are rare proportionally to other fossil remains.
Have we seen a dinosaur? IDK, I haven't seen one. Some people have claimed to see them. Are they valid? IDK. It is interesting to read descriptions of human contact with large beasts before the invention of the word "dinosaur" in 1842 and before the discovery of the first dinosaur fossils. Rhetorically, what would accounts read like if a species was going out of existence? Would we believe those accounts if we were living but had never seen the species ourselves? IDK.
That was likely me. Ever been to someplace like the Carlsbad Caverns and looked at the formations created by deposits of micro thin layers of dissolved limestone? If you have a slight understanding of the process and have observed the build rate, it isn't hard to imagine that all that didn't happen in a few thousand years. If you understand erosion and how it works, it isn't difficult to imagine that something like the Grand Canyon didn't happen in a few thousand years. My point was if you have an open mind and some basic understanding of nature, it is impossible to believe that the earth is only 5000 years old.
Yes, I've been to several caves, Longhorn Caves, Sonora Caves, all in central TX. I understand build rates, they are proportional to the amount of water that seeps through the ground. If that is a constant, then we can back calculate to get an estimated age. However, no one was there to observe if they were constant from the beginning of formation. So, it is less credible/certain, than an observable phenomena from the onset.
Erosion is another process that we can measure a present rate, but whether that rate was the same in the distant past is unknown. Did a breached dam occur via overtopping or piping that played a role in the formation of the Grand Canyon? There is evidence of an ancient lake known as Hopi Lake in NE Arizona. These processes would be adequate for cavitation and plucking, two catastrophic geological processes that we can observe today and may employ to explain the formation of Grand Canyon. Just saying, everything is not as cut and clean when scientists give explanations of distant past formations, etc...as they are when doing scientific experiments in a modern laboratory.
In short, if the present is the key to the past, we don't have any problem unlocking it. But how do we know? Nobody was there to observe it and take a measurement. If intellectually honest, which some are, it is a guess.