How were the Japanese who pretended to be China people to spy on information discovered?

1. Language: Japanese who have not stayed in China for a long time cannot speak Chinese fluently. Chinese is too difficult. I met a Japanese whose wife is from China. She has been married for ten years and has no difficulty in daily communication, but she doesn't understand idioms.

2. Habits: Unless you are an old hand and have been specially trained, the typical habits of Japanese people cannot be changed, such as nodding, nodding, bowing and being polite all the time.

3. Behavior: Since you are a spy, you will not be interested in ordinary things, and you will certainly try your best to get close to sensitive targets, which will also expose yourself greatly.

in addition to the above, there are some interesting identification methods, but they are not reliable except for being funny.

For example, Japanese people wear clogs, which will lead to flat feet. This statement may have worked before World War II, but now it is basically a joke because of the change of Japanese lifestyle; And let the Japanese eat exotic foods such as stinky tofu, which is even more unreliable. The Japanese are really picky about food, but I have seen many people in China who don't eat stinky tofu, and I have seen Japanese who eat stinky tofu too much. For a Japanese spy, if eating stinky tofu can cover himself from exposure, will he refuse?