The hallmarks decide it. On a pen with a solid gold body every gold part carries its own fineness hallmark — clip, cap and barrel each separately — usually marked 750 for 18 carat or 585 for 14 carat. Where such a mark is missing, the part in question is not solid gold but gilt or plated. In short: what is gold is also stamped.
Rolled gold and doublé versions carry at most a maker's or material marking instead of a fineness figure. Since the surfaces of the two are hard to tell apart in good condition while the metal value differs enormously, always look for the stamps yourself before buying rather than relying on a description that merely says “gold”. The pattern of wear gives a further clue: on plated pens the gold layer rubs through in time at the cap lip, the clip and the edges, letting the base metal show through, whereas solid gold simply dulls evenly.