You know what's hard? Following the Lord.
How do you know what you are hearing is HIS voice? Well first we need to be careful to do ALL His commandments as we are told in Deuteronomy 28:1. We also need to be sure to be practicing wisdom ( Ps. 111:10) so that we have a good understanding of Him. But certainly, we need to remember, that we have a perfect example in Christ. We have all been called and His example is there, in the Word, for us to follow in His footsteps (I Pet. 2:21).
In order for us to hear Him we need to know Him. An important step to help knowing Him include being in His Word -- daily. So each day we are to get to know a little bit more about Him, His character, His desires, His rules. Each word in the Scriptures are made for us to know just one more tidbit about Him to help us become like Him. One way to read the Scriptures and study them is by using what is called S.O.A.P. (Scripture, Observation, Application, and then Prayer). We have just started this journey at our church as a whole. We have started our study in I Timothy and soon our sermon series will be on the same book of Scripture.
So each day as we take time reading the Word and then trying to apply it to our every day life we are learning more and more about Him and HIS ways. This is how you know HIS voice - by spending time with Him, learning about Him, and even doing what He says -- feeding the poor, taking care of the widow, teaching those around you who you work with and live near about Him by what you DO - not what you say.
Be sure to take some time today ... read with Him, be in His Word so you can HEAR Him speak to you with His voice.