Great question. I often think about this. Due to the birth of my 2nd child (2 years ago) and a near fatal injury 3 months ago that I survived by the grace of God, my faith has increased 1000 times. I pray several times each day and I often ask the Lord to guide me in the direction he wills me to go for various issues.
Now I know the question you are asking most likely doesn’t have one solid, indisputable answer. So, I will tell you what I have personally experienced and other forum members will give their insight. What I have found is that the Lord has answered almost every one of my prayers (sometimes in very ingenious, unexpected ways) when I ask Him for something that He wills in the first place (ie: to get along in a mutually cordial way with someone I don’t get along with).
While praying recently, I asked God to give me some creative ideas for teaching the Catholic faith and traditions to my 5-year old daughter and 2-year old son. Within minutes, I had 3 awesome ideas pop into my head, 2 of which I really hadn’t considered before.
I think you tend to know the difference between your voice and God’s voice by the type of information that enters your mind when you pray to Him. If it’s an idea you have thought of before, it’s likely your voice. If it’s an entirely new idea or a unique twist on one of your old ideas, it’s probably God’s voice.