Gold marks on pens
Although pens for the US market did not have to have their precious metal content marked by law, solid gold pens and pen trim are almost always going to be marked usually on each piece. Silver will generally be marked as well. Note that karat markings followed by a fraction indicate a laminate (gold filled or rolled gold), in which the fraction indicates the proportion of gold by weight.
Elsewhere, government standards required official testing and marking, making it easy to determine the material and the date of the piece in question (and also sometimes the maker: FDW, for example, was the English mark for Waterman). Watch out, however, for the many European pens with clips marked 585 (the decimal equivalent of 14k gold): if the other parts of the pen aren't also marked, you can be sure that they are gold filled and not solid gold. It is usually very simple to determine if a pen or pencil is gold filled using only a strong magnifier under good light. On all but near-pristine examples, gold filled objects will display telltale wear-through to the underlying layer of brass or copper. Look carefully at vulnerable high points and exposed edges.
NOTE: often one will run across a pen or pencil that the seller swears is solid gold ("it's been tested by a jeweler", they'll say), yet which bears no marks. Beware: older gold filled items have a very thick surface layer of gold, thick enough to register as solid when tested.