When doing a catering job on shabbos, if I see the goyim loading the truck on their own do I have an obligation to stop them?
Yes you do. If a gentile is doing work in public (like loading up a truck) and it appears that he is doing it for the Jew, even if the gentile is really doing it for himself, the Jew may not allow him to continue, and he has to ask him to stop. If the gentile, is actually doing it for himself, (he was hired to do the whole transporting job, and it is his responsibility to get the equipment to the simcha and back, and he can do it whenever he likes), and you already told him not to do it on Shabbos, then you don’t have to any more than honesty telling that you don’t want him doing this work on Shabbos.
O:CH 244-1,3, M:B 244-22.