As a security measure, Apple provides a list of devices that you’ve signin using your iCloud account. From iOS, go to Settings > iCloud > Tap your account name > Devices.
Check if any of the device on the list is unknown to you. If there is you should remove the device immediately and change your iCloud password.
The list might contain your old iPhones, iPads, Apple TV, Macs that you’ve owned over the years. Remove those devices you no longer using or sold. Cleaning out the list will make it easy for you to spot any new device with unauthorised signin. Simply tap the button “Remove from Account” to remove the device.
You can access the same device list on the Mac under System Preferences > iCloud > Account Details > Devices.