It is all depend on your browser history either from Desktop / Laptop or Smartphone. For example you would have visited a website and signed up with laptop browser, in case if you have accepted the option of 'save user id / password' then the browser could have saved it.
If not, you could have remember it or written it or the general one which you always remember (standard user id / password). Thus, you can go to the website listed one or wherever you signed up and then try to login to delete the account.
In Google Chrome (Desktop or Laptop), type URL as chrome://settings/passwords and enter it. You will find the list, website name, user name and password etc. You can view the password even if it is dot password by clicking 'eye' logo / image.
In Safari (Desktop or Laptop), try to click Safari then Preferences. Click the Passwords tab and enter your Mac's password, click on an item to view the password. You can double click on user name to view it.
If you are using Android (Smartphone), select the browser, for example, chrome or opera, you can select 'setting' option at the top right of the chrome browser, you can then click on 'Passwords' to view the list of signed up and password you have done it.
If you are using iPhone (iOS), you can find it in the setting itself. Again, almost same procedure as long as you remember iPhone user ID and Password. Or else, you have to select the 'forget password' option to get things done.
Update: Once you logged in at the old websites where you signed up earlier and would not like to continue with them, you can then go to setting for 'deleting your account permanently'. That's it. You're done.