This guide's full text is currently available in English.
Start from your inbox, not from a search engine
Dormant accounts are usually discoverable from the one place you cannot avoid: your email. Search your mailbox for the word 'welcome', 'verify', 'confirm your account' and 'password reset', and sort by sender.
That search reconstructs a list of services that once considered you a user. It is more complete than any username scan, because it includes the accounts you made with a throwaway address.
Then sweep the handles
Take the two or three usernames you have reused over the years and check each one across platforms. Anything that resolves is either an account you still control or one you abandoned without closing.
Run the same sweep on the email-derived list. Where a service still exists and shows a profile, decide immediately whether to keep it, sanitise it, or request deletion.
Password managers and browser profiles as inventories
Your password manager is an inventory you already paid for. Export the site list and read it: it is the most reliable record of everything you signed up for and forgot.
Browser autofill and saved-login lists catch the remainder — the throwaway signups you never stored anywhere.
Close the loop on what you find
For each account: export anything you want to keep, then request deletion rather than simply logging out. Deactivating leaves the profile and the data in place.
Expect lag. Some services honour deletion within days, some keep the profile visible for months, and a few retain backups long after the profile disappears. Note which ones you have dealt with so you can re-check.