I do not that there were any “arms” on the Lusitania. There were a number of Canadian Army Officers, out of uniform, who boarded in New York, which was a violation of U.S. neutrality, and which the U.S. winked at, and did not take notice of. There was also a small cargo of “gun cotton” which would probably be used to manufacture artillery propellant in England, and which could be classed as a munition. American passengers would have been unaware of these violations of neutrality. As was the submarine that sank the Lusitania without warning.
But U.S. passengers could not have been unaware of the advertisements paid for by the German embassy in the shipping pages of New York newspapers warning that the German government could not insure the safety of the Lusitania specifically. Anyone looking for sailing information of the Lusitania in New York newspapers, would have seen those advertisements.
The Lusitania carried 4000 cases of machine gun ammo, and a large cargo of 15 pounder shells and other military cargo. It’s been proven by examinations at the wreck, complete with samples and photos.