I know that this has been asked a lot, but how can you tell if someone is looking at your Facebook?

You can't.  Not really.  the only way I know if someone has looked at my posts or my timeline is when they click a 'like' button or when they comment.  
Let me also say, that Social Media is not what it should be. People have become too involved in who is looking, who is liking, how many likes did a photo get, and comments, oh geez, 'What does it mean when they say this, or say that, or delete  comment, or unfriend me, Oh my heavens.  Stop putting so much of your life in what happens on Social Media.  it's not real, it's not a true indicator of who you are or what you are., or what people truly feel about you.   

not possible any apps that claim that they can find this out are fake don't
work and used towards spam