The Washington Naval Treaty, also known as the Five-Power Treaty, was a treaty signed during 1922 among the major Allies of World War I, which agreed to prevent an arms race by limiting naval construction. So how did they actually control and enforce it? did they make some kind of organization to monitor every ship that gonna be build or did They have to report every ship that gonna be build?
In general, the Washington Naval Treaty (and the other naval treaties of the 1920s and 30s) was poorly enforced by the signatories. Nations generally regarded mutual inspection as an unconscionable act, one that would require too much interference in their sovereign affairs. It would violate their sovereignty, and open the door to further actions with a greater implication for military intelligence. As such, the regime imposed by Washington relied on self-reporting of tonnages by the signatories. To provide some degree of verification for the tonnages, they were also required to publish the basic dimensions of the ships they were constructing (the length, beam, draught, and armament). From these parameters, the naval engineers in each fleet's design teams could estimate the tonnage of the ships. However, this was difficult to do, and relied on the published figures being correct. Other nations could verify these claims clandestinely using their intelligence agencies - one of the most important jobs for the RN's Naval Intelligence Division in the interwar period was detecting cheating in terms of tonnages - but this gave little chance for recourse. In theory, there was a strong incentive not to cheat; if you cheated on the treaty, then it would give your enemies leave to break the treaty, removing your advantage.
As you might expect, this lax environment gave plenty of opportunities for cheating. Every signatory to the Washington Treaty took advantage of them, to a greater or lesser extent. The British and French did so to a low extent, by under-reporting the amount of food, fuel and ammunition carried as part of the standard displacement. The Americans went somewhat further, claiming that only equipment developed at the time of the Washington Treaty counted as part of the standard displacement. The Italians and Japanese (and Germans in the 1930s treaties), cheated shamelessly, lying about the full displacement of the ships they were producing. They lied about the tonnage of the ships they were building, and to conceal this lying, lied about the dimensions of those ships.
An example of how enforcement of the treaties worked (or didn't) came with the Gorizia-class heavy cruisers, built by the Italians. The Washington Treaty required that heavy cruisers be armed with 8in guns, and to displace no more than 10,000 tons. In actual fact, the Gorizia class were about 1,000 tons over the limit. In 1927, when the ships were under construction, an Italian engineer informed NID of the true tonnage of the class, but there was little corroborating evidence. This would not come until 1936, when Gorizia was damaged in a storm off the coast of Spain. She was forced to seek repairs in Gibraltar, using the British shipyard there. While she was there, the British engineers measured her dimensions, looked at the ship's plans, and estimated the thickness of her armour and the weight of her stores. This gave the RN the evidence it needed to conclude that the Italian ships were over the tonnage limit. The British made official protests, but to little avail. This was an unusual situation, as it relied on the British had privileged access to the ship in question. More usual was the RN's estimation of the true displacements of foreign battleships. In July 1936, Captain Tom Phillips, Director of the Plans Division, attempted to estimate the displacement of other navy's battleships by use of a weight coefficient. He took the known displacements and dimensions of British battleships, estimating a density for a typical battleship. From this, he could use the dimensions of the foreign battleships to estimate their true displacements. According to this, the French, Germans and Italians were all understating the displacements of their ships by 3,000 tons. This was an overestimate for the French, but underestimates for the Germans and Italians (not surprising given that the Germans were underreporting the draughts of their battleships). This was a rather crude estimate, so the British could not act upon it. The RN's Chief Constructor, Stanley Goodall, also advised that this might not be evidence of duplicity, as some degree of error in calculating tonnages was to be expected, with a 3-5% error being considered usual.