how can i know my facebook account hacked or not?

[Yogendra Chavda]

You can check your login history for that. Who logged in and from where so that you can track your facebook account's activity.

Do this following steps to enable this:
Click the down arrow next to "Home"
Click account settings.
Go to the security tab
Click login notifications.
Make sure it is checked.
If it is, you'll need to enter a device name every time you log in, the time of the login is recorded, as well as the external IP address of whoever is using it. This will be sent to you in an email, do check there often.

Hope this helps.