A dishonest steel salesperson can also lie to you about when their steel is from. If you don't have a way of measuring the radiation, then you have to rely on trusted sources that you don't believe will lie to you, and not accept any steel from someone you don't know.

It's the same here, we have many trusted sources that we don't believe would lie about when content is from - Internet Archive, newspaper archives, widely used web crawls, Wikipedia exports, etc. Obviously you don't use an AI (or a black box search engine) to get non-AI content, you get it from people and systems you trust.