How do you know it is God speaking to you?

How do you know it is God speaking to you?

Let's start off by saying the simple answer: it always aligns with the Word of God. The Lord will never speak to you in a way that contradicts His written Word. His written Word is the litmus test for all of what we believe Him to be speaking. Now, that is not to say that everything you hear Him say is written is Scripture. For example, if you sense the Lord speaking to you about encouraging another believer with a text or card that is not necessarily in the lines of Scripture but it aligns with Hebrews exhorting us to encourage one another.
 
The more extended answer would delve into how we hear the Lord speak. The Holy Spirit can speak to us through almost any means He chooses. Because believers are indwelt by the Holy Spirit, He can use all things to illuminate our hearts and minds. If you are searching for God to speak, the best places to seek His voice are: His word (written and preached), counsel from godly people, and His creation.