We can tell by how the continents look. Scientists who propose the pangea theory have said this. You can see for yourself because all the continents look like puzzle pieces you could put together. Scientists can also tell from the species. For example, let's say a scientist found a bone of a creature at the edge of a continent, the edge of another would have the same species. They have also calculated that the continents in itself are still splitting apart, but very slowly. The reason they split up was because tectonic plate movements.
