The justification (if it can be called that) for Internment wasn't identification of any one individual spy, but rather the suspicion that anyone of Japanese descent would be a member of the "Fifth Column" for Japan – and engage in activities that would be treasonable during the war, including espionage.

This belief was primarily formed from a combination of two separate forces – the first and most obvious was the long running racist anti-Asiatic prejudices that would characterize the war with Japan. The second was the fear around "Fifth Columnists."

Fifth columns became a source of hysteria in late 30s as the fascist governments in Europe used minority groups to subvert foreign governments - for example the Sudetendeutsches Freikorps was formed at Hitler's express order and engaged in an escalation of violence that was used by Hitler to "justify" his seizure of the Sudetenland and later the subjugation of rest of Czechoslovakia. The term itself came into popularity in the Spanish civil war, with the Nationalists talking about "fifth columns" that would disrupt republican held cities and government.

With that in mind, let's break apart the question:

1. What spies did Japan have in the west coast prior the outbreak of war?
2. What about crimes other than espionage?
3. Was it worth the trouble of capturing and detaining citizens in the camps?

What spies did Japan have in the west coast prior the outbreak of war?

While common perception is that nations deploy large systematic and highly organized efforts to spy on foreign countries, the truth is that most espionage, especially during this period, was much smaller in scope and grandeur. The chief Japanese Spy on the west coast was Takeo Yoshikawa. Like most military espionage at this point, Takeo Yoshikawa's cover was working at the Japanese Consulate in Oahu, and for the most part he engaged in legal spying - using the fact that there was a large Japanese population cover his interests. That doesn't mean that he found the Japanese community helpful - he complained that they were loyal to America first, and not to Japan. He used the population to blend in but was doing completely legal activity such as tourist flights, and diving. He just happened to be looking for military information. This freedom of action allows Takeo Yoshikawa to inform the Japanese the day before the attack that the carriers were not present in Pearl Harbor. The most helpful contact he had was probably Otto Kuehn – a German spy, who was arrested in Hawaii in February 1942.

Outside of Yoshikawa, the value of Japanese spies go down dramatically. Miyazaki Toshio was posed as a student at Stanford University in the states, but ironically his one success was recruiting Harry Thompson to spy on the fleet at San Pedro. This was discovered by the Americans, and Toshio fled back to Japan before the start of the war.

More espionage on the fleet occurred around San Diego – but on the south side of the border. Japan had a network that’s main focus was the American Fleet in San Diego operating from a Japanese owned brothel in Tijuana. Finally Japan also had the famous “Doll Lady” Velvalee Dickinson , who passed on information via steganographic letters to contacts in South America.

While Japanese encrypted cables (that the United States was decrypting and reading) boasted that there were large populations that would act as fifth columnists, while there was an effort by Japan to actually leverage these people, there is no evidence that anything of more than trivial value was ever provided by Nisei or Issei on the west coast.

What about crimes other than espionage?

Unfortunately, the official American report that made the claim “not a single documented act of espionage, sabotage or fifth column activity” is flat out untrue. Immediately after the attack on Pearl Harbor, one of the pilots of the Pearl Harbor attack crash landed on the Hawaiian Island of Niihau. Niihau was run as a cultural preserve by the Robinson family which closed the island off to tourists and encouraged native Polynesian lifestyles. Therefore, there was no regular power and limited access to radio. While the pilot was stunned, one of the native Hawaiians sized papers from the airplane before the pilot could destroy them. The pilot communicated with three Japanese who lived on the island, turned on the Polynesians in an effort to recover the codes and maps taken from him, but was ultimately killed by the Polynesian, while one of the Japanese committed suicide.

That said, outside of this incident, I am not aware of any other effort to give material aid to Japan by anyone who was removed from the west coast.

Was it worth the trouble of capturing and detaining citizens in the camps?

This is unquestionably a moral judgement – but there is fairly strong consensus that the interment was a massive failure of democracy and completely unwarranted. This verdict is likely to hold, but there are some things that historians and commentators still discuss:

1. It’s not quite as cut and dry as the Commission on the Wartime Relocation and Internment of civilians makes it in their report which led to reparations from the United States Government. For example, the commission ignored the Niihau incident with the exception of a single footnote, and deliberately ignored magic cables where the Japanese made a number of claims of support from Issei and Nisei populations
2. The most decisive battle of the pacific war is generally held to be the battle of midway. The battle of midway depended on the United States surprising the Japanese invasion and carrier forces with the Hawaii based carrier forces. Virtually anyone on the island of Hawaii could determine if the carriers where at Pearl (as the Japanese believed) or not. That knowledge would have almost certainly changed the outcome of the battle. The entire Japanese purpose was to draw the American carriers into open combat, only the element of surprise led to the American victory.

Would not having the internment change either of these things? Probably not. The Japanese didn’t have any shortwave equipment on Hawaii prior to the war. Even the report of which ships were present in pearl harbor on December 6th went through RCA. This communication channel wasn’t available to them. Even if some spy was able to determine the fleets location, they would have no way to communicate it.

The value of the internment is questionable at best. This was reflected in the Ringle Report – which was commissioned before the war started and finished shortly thereafter found that: “only a small percentage of Japanese Americans posed a potential security threat, and that the most dangerous were already known or in custody.”

Some good resources:

https://archive.org/stream/Personal-Justice-Denied/PersonalJusticeDenied-Report_djvu.txt (Official US report on Internment)

https://www.justice.gov/archives/opa/blog/confession-error-solicitor-generals-mistakes-during-japanese-american-internment-cases (Talks about the Ringle report)

https://internmentarchives.com/showdoc.php?docid=00416 (good break down of the magic communications around the Japanese population in the united states)

https://www.amazon.com/Pearl-Harbor-Greatness-Craig-Nelson-ebook/dp/B0176M40DM/ref=sr_1_2 (recent Pearl Harbor book which goes into some detail on Yoshikawa)

https://www.amazon.com/Broken-Seal-Operation-Harbor-Disaster (goes into the various espionage going on before pearl harbor)

https://www.amazon.com/Hawaii-Under-Rising-Sun-Conquest/dp/0824825500 (talks about the post-midway invasion plan for Hawaii)

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and interrogated people

Was there any sort of interrogation process or other attempt to sort out whether any detained person was a spy or even sympathetic to the Japanese Empire?