If you are suspecting that someone has access to your Facebook account, you can configure your account security alerts, which allow to send alerts by email whenever someone signs in to your account.
1. Sign in to your Facebook account and head to Account >>Account Settings.
2. Click on change for the Account Security option.
3. Check on "Send me an email," then click Save.
4. On your next access to your account, you will be required to add a name for your computer so as to add it to the trusted list of devices, then hit Continue.
5. From now on, if someone connects to your Facebook account, you will receive an email containing its IP address, location, time of connection, etc.