This is a really interesting question because understanding how sceintists get information is a key idea in understanding how science actually works. Scientists use a process of thinking called the scientific method in order to test their ideas. The first step is to come up with a question, for example we’ll ask: is global warming happening? So scientists will start gathering background information until they feel comfortable constructing a hypothesis. A hypothesis is a testable statement that the scientist believes to be true and can serve as the foundation for experiments. The hypothesis must be testable because science is based on observed data and scientists do not assume anything to be true until it is rigorously tested. So, for the sake of our example, let’s say that our hypothesis is that Earth’s average temperature is rising at a higher rate than usual. Now that we have a hypothesis, we craft an experiment that will either prove or disprove our hypothesis. Let’s record the average temperature in our home town every day over the next ten years, and compare that to historical average temperatures. If we ran the experiment and gathered all the data we could analyze it and see if our hypothesis was correct. If it was then we can use that data to predict further changes in Earth’s temperature, but if it wasn’t correct then we need to go back and alter our hypothesis and start over again. Scientists use this model in order to gain all sorts of information whether it be about changes in the Earth or our cells. Thank you for the question!

Anything that happens on Earth leaves a record in the rocks: sandstone is the remains of sands that were once sediment in a river or ocean, lava is the remains of past volcanic eruptions, fossils are the remains of once-living things that got buried and preserved. Additionally, there are some simple logical rules as well: if you lay some layer of sediment or rock down from the water, then it will be lying on top of whatever was underneath it, and, therefore, younger sediments will be on top of older sediments.

The predictions come from what we call the "principle of uniformitarianism", which states that the processes acting on the earth in the past are the same as the processes acting on the earth today and are the same as the processes that will act on the earth in the future. This is an assumption that we make, indeed that all science makes, which is that the universe has rules that describe how it behaves. For instance, if we look at the debris from a flood that happened yesterday, and then we look at debris from the past that look the same, we can infer that the previous debris were created in a flood. If we look through the record and see that floods of some scale happen every thousand years or so, then we can predict that they will continue to do so into the future.