Question

I was wondering how on the face of this earth do you find known isotopes

Answer 1

In what sense? Do you mean, how do you separate isotopes out of a particular sample of an element? If so, you use the physical differences between the isotopes to isolate them. The primary difference between, say, U-235 and U-238 is the mass of individual atoms. One way to separate them is to use a centrifuge. You vaporize the uranium, then spin the gas very quickly. The more massive atoms of U-238 tend to move further out from the center than the atoms of U-235, so you can bleed off gas that has a higher proportion of U-238 from the rim of the centrifuge.