Paste a random substring and ask it to autocomplete the next few sentences. If it's the same and your temperature > 0.4 then it's basically guaranteed to be a real system prompt because the probability of that happening is very low.

See https://news.ycombinator.com/item?id=43911687 .

Come back in a few months to see this repo taken down by Anthropic.

It’s already down. Did you happen to save it? I’m just coming across it

Ask the Anthropic people