You can detect if someone is using iCloud Private Relay super easily

Apple publishes a database of their egress IP addresses and the entire world's iCloud Private Relay traffic exits through ~105K IPv4 addresses.

I analyzed it and found some surprising stuff. Let's dig in: The US is the least privacy-safe country to use iCloud Private Relay in the world (!!!). Italy's Relay users get ~7x more hidden/shared traffic per IP than US users. Back of the envelope:

45k 🇺🇸 IPv4s shared among ~31M iCloud+ users = 683 users per IP

Most ➜ least private:

🇮🇹 4,697 users per IP
🇳🇱 4,115 users per IP
🇫🇷 3,929 users per IP
🇯🇵 3,647 users per IP
🇬🇧 2,965 users per IP
🇮🇳 2,629 users per IP
🇩🇪 2,623 users per IP
🇧🇷 2,169 users per IP
🇨🇦 1,485 users per IP
🇸🇬 1,051 users per IP
🇫🇮 967 users per IP
🇺🇸 683 users per IP

When you narrow it down to time-of-day traffic it can get a lot more specific than that, too. All estimates, though.