Review textbooks in the field. Texts will usually specifically reference research that significantly advanced the field.
Follow the chain of references from current papers in the field. Each paper will reference the work of others who preceded the current authors. When you find a paper of interest, see whom they reference as doing the preliminary work, then do the same thing with those references. It usually takes only a couple of steps to identify a few papers that have broken new ground.
Track down papers that have been heavily cited. If a single paper has been referenced by many different authors, it's likely the work was significant. The most helpful resource may be an online journal index, although you may have to buy access. In that case it's more economical to visit a university library and get access while there. If you can't get access, you can find the same information through all-purpose Internet search engines, just not as well organized.
Talk to people working in the field. This is an essential step. You can miss many significant papers in the other steps, either because the research is too new or because it was not recognized as important for some years after it was published.