Western blot analysis can detect a specific protein in a mixture of any number of proteins while giving you information about the size of the protein. It is able to do this based on the use of a specific monoclonal antibody or polyclonal antibody. It does not matter whether the protein has been synthesized in vitro or extracted from cells in vivo or ex vivo. This method is, however, dependent on the use of a high-quality antibody directed against a desired protein. Thus, you must be able to produce at least a small portion of the protein from a cloned DNA fragment or purified protein which is then injected into a mouse or rabbit to generate a monoclonal antibody or a polyclonal antibody respectively. You will use this antibody as a probe to detect the protein of interest.
Western blotting tells you how much protein has accumulated in cells. If you are interested in the rate of synthesis of a protein, Radio-immunoprecipitation (RIP) may be the best assay for you. Also, if a protein is degraded quickly, Western blotting won't detect it well; you'll need to use (RIP). See the section on RIP for more information, as well as a helpful comparative chart that illustrates the differences between these two techniques.