Habakkuk 2:1-2: "I will stand on my guard post and station myself on the rampart, and I will keep watch to see what He will speak to me...Then the Lord answered me and said, 'Record the vision...'"
Ways to be sure what you're hearing is from Him:
TEST THE ORIGIN (1 John 4:1)
Thoughts from our own minds are progressive, with one thought leading to the next. Thoughts from the Holy Spirit are spontaneous. The Hebrew word for true prophecy is "naba", which literally means to bubble up, whereas false prophecy is "ziyd" meaning to boil up. True words from the Lord will bubble up from our innermost being; we don't need to cook them up ourselves.
COMPARE IT TO BIBLICAL PRINCIPLES
God will never say something to you personally which is contrary to His universal revelation as expressed in the Scriptures. If the Bible clearly states that something is a sin, no amount of journaling can make it right. Much of what you journal about will not be specifically addressed in the Bible, however, so an understanding of Biblical principles is also needed.
COMPARE IT TO THE NAMES AND CARACTER OF GOD REVEALED IN THE BIBLE
Anything God says to you will be in harmony with His essential nature. Journaling will help you get to know God personally, but knowing what the Bible says about Him will help you discern what words are from Him. Make sure the tenor of your journaling lines up with the character of God as described in the names of the Father, Son, and Holy Spirit.
TEST THE FRUIT (Matthew 7:15-20)
What effect does what you are hearing have on your soul and your spirit? Words from the Lord will quicken your faith and increase your love, peace, and joy. They will stimulate a sense of humility within you as you become more aware of Who God is and who you are. On the other hand, any words you receive which cause you to fear or doubt, which bring you into confusion or anxiety, or which stroke your ego must be immediately rebuked and rejected as lies of the enemy.
SHARE IT WITH YOUR SPIRITUAL COUNSELORS (Proverbs 11:14)
We are members of a Body! A cord of three strands is not easily broken, and God's intention has always been for us to grow together. Nothing will increase your faith in your ability to hear from God like having it confirmed by two or three other people who have a solid Biblical orientation.