If you want to look for techniques, there are lots under the umbrella of OSINT. It stands for Open Source Intelligence, but it doesn't refer to open-source as i

If you want to look for techniques, there are lots under the umbrella of OSINT. It stands for Open Source Intelligence, but it doesn't refer to open-source as in software. It just means the sources of information are public instead of being secret. Sometimes you can just Google the person's username, or check the WHOIS records on their website.