Gustave Doré, Depiction of Satan, the antagonist of John Milton's Paradise Lost c. 1866 (Photo credit: Wikipedia)
You can be influenced by demons. Even if you have been saved and you are a christian. As a matter of fact Christians or believers get attacked and tempted more so than nonbelievers of God.
So yes there is both evil and good spirits that surround us all. The best way to tell is listen to the voices in your head.
There is 3 voices yours,Gods and demons. God never tells you to hurt yourself or others. God never tells you negative things like your worthless for example. If you are hearing your worthless. It could only be one voice or the other and thats yours or Satan and his demons.
Pay attention to the sound of your voice when you speak to yourself in your mind. Then listen to the voice that is telling you your worthless or worse.