How does SynthID detect whether a text is watermarked through the API?

Gemini API
,
1

I saw a tool for adding watermarks to text and detecting watermarked text at SynthID：用于为 LLM 生成的文本添加水印和检测此类文本的工具  |  Responsible Generative AI Toolkit  |  Google AI for Developers

I learned from the SynthID - Google DeepMind page that there is an entry.

But I cannot verify whether the results on the SynthID - Google DeepMind page are watermarked