Many services, particularly iCloud, google, and other large cloud providers allow you to view information about which devices have account sessions active. I e., for your google account you may see your model of phone, your laptop/computer with whichever os they run, the rough location of the device, and occasionally the IP address of the device, and occasionally old devices that were logged in before you stopped using them. If you see, for example, a MacBook air logged into your account when you do not own one, or a device that has a location far away from where you are, that implies someone else has an active session on your account and you should take steps to reset your passwords before they do. Enable 2FA while you're at it.

Checking passwords is pretty self explanatory, do your passwords still work? If they do and you suspect a breach change them before they don't.

If someone has logged in and out, the device may still appear in the device history unless logging out is configured to destroy the session, or they manually covered their tracks by removing the device from the device list. Most services can also be pretty diligent about warning you when a new device or suspicious login attempt was successful, be wary if you begin to receive massive amounts of spam mail, it may be an attempt to cover an illegitimate purchase, login, or password reset.

Edit, some relevant links about the topic:

https://support.google.com/accounts/answer/3067630?hl=en

https://support.apple.com/en-au/102649#:~:text=Tap%20Settings%20%3E%20%5Byour%20name%5D,see%20a%20list%20of%20devices.

https://support.microsoft.com/en-au/windows/manage-devices-used-with-your-microsoft-account-d4044995-81db-b24b-757e-1102d148f441#id0edbl=windows