we can determine the age of rocks using radioactive decay. it is a sort of clock. hence we can determine the shape of the land and where the various land masses were as a function of time going back SEVERAL BILLION YEARS at least !!!! amazing but true... for example, we know that 200 Myrs ago there were a string of large volcanoes towering up to 20 000 feet above what is now the SIERRA NEVADA mountains in eastern CALIF

*Hi Melissa, There are different kinds of ways that you can establish that changed has occurred on earth. One way is from space. You see the changes in forest extent in Brazil, for example, by comparing (spatially) a satellite image over Brazil in 1975 and a second image over the same area in 1999. You can see deforestation, changes in plants that live there, et cetera. That is kind of a quick change. Scientists can also see change over much longer periods of time and an example of this type of change is called carbon dating - you can find lots of references to that on the web if you do a search. Carbon dating can tell you if a bone belongs to a dinosaur or a dog that was buried only a few years ago.
Jennifer Gebelein, ucsb, Geography

One method that I assume scientists use to see how things were in the past and compare them to today is to take a core sample. This means drilling into the ground in an area where a lot of sediment has built up over time (like the bottom of a dry lake?) and taking out a thin cylinder that is as long as possible. This cylinder would be made up of layer upon layer of dirt and organic matter that settled on stuff that fell on that spot before and is then covered by stuff that comes later. You can look at the chemical composition of various layers to guess at how things may have been in the past and changed over time. An abrupt change in conditions of the planet should cause a noticeable change in the sediment layer. Using radioactive carbon dating, you can estimate the age of each layer and develop a time line. How to use
radioactive carbon dating is a whole other question which you should feel free to ask about if you can't find a good explanation.

Scientists also can take core samples from very old trees. They use the tree rings to count back the years and the thickness of the ring to guess at the climate that year. This method can only go back hundreds of years while the previous method can go back hundreds of millions of years.