What's the cleanest way to test 'my llms.txt actually works'?

Hypothesis: AI engines reading my llms.txt should describe my product using terminology from that file.

Test:

1. I added a distinctive phrase only in llms.txt, never on the homepage or anywhere else: '<a slightly unusual phrase that perfectly describes my product>'
2. Two weeks later, query ChatGPT / Perplexity / Google AI Mode with 'what does <my brand> do?'
3. See if any response includes that distinctive phrase.

Results after 14 days:

• Perplexity: yes, used the phrase verbatim in 2 out of 3 runs
• ChatGPT: no, used generic category language
• Google AI Mode: no, did not pick up llms.txt content at all

So my interim conclusion is llms.txt is mostly a Perplexity signal today. ChatGPT may or may not be reading it. Google AI Mode definitely isn't (yet).

Better testing ideas welcome.