Sometimes it is difficult to tell. If you have your mind set on God's word, you will be much more likely to make right decisions.
See Joshua 1:8,9; Psalm 119:9; Proverbs 3:5,6.
Here is a link to information on the website which will help you with this question:
The Bible Our Guide (please click on the title)
I hope you have found this helpful.
God bless,
Mike