I don't think this is true, depending on the task they can be extremely hard to spot. You especially don't want to accuse a student of cheating using AI without very concrete evidence.

Students are cheating by using a program that can do their homework for them.

A smart professor hid a guideline to cite works by a dog.

The students who copy pasted the prompt got works attributed to a dog in their homework.

A lot of students have started using ChatGPT to write papers for them. This person is saying they leave directions for an AI text generator in their directions that are hidden from view but which would be observable to the AI scanning it. So any paper turned in with that specific alteration would be almost certainly from a cheater.

The professor hides white text on a white background to catch potential cheaters. The actual assignment is written in black text. If the student has followed the instructions that are written in white, this is a good indication that they may have cheated, because human eyes won't see the white text against a white background, while a computer program writing a paper for the student will see the white text and follow the additional instructions.