Scientists can use the color spectrum from distant objects. When looking at the color spectrum, gaps appear. Based on the gaps, scientists can determine what elements are present. Have they all been found? Who knows.
"Peaks and dips at specific points in a spectrum indicate the presence of elements like carbon, oxygen, hydrogen, iron, and more. That information can be used to make other conclusions about the object. Stars that contain just hydrogen and helium, for example, formed early in the universe. Stars with heavier elements formed later, after their predecessors exploded and seeded the universe with those elements."
http://hubblesite.org/laserart/spectra.php
Yeah! Science bitch!