God is sovereign and he can speak to anyone, anytime, anyway He chooses.
How do we know when it is God who is speaking to us? My experience has been that when God has spoken to me in a manner that I clearly understood what He was saying, the fact that it was God who was speaking was self evident. The authority of the voice was such that there was no question as to who was speaking, and there was no question but what the voice must be obeyed.
I first heard God speak to me very clearly when I was twelve years old. I was not a Christian, and had no Christian upbringing, and I was very much caught by surprise. But the voice was so firm and authoritative that my decision to do as he told me was a very easy choice to make.
On another occasion, after I had been saved, I was riding a city bus with a friend, watching my watch, because my friend and I were hoping to get to the buffet before 4:00 p.m. because at 4:00 p.m. they stopped serving lunch and began serving dinner, and dinner was much more expensive than lunch and neither of us could afford the dinner price. But the bus kept stopping to load or unload passengers and I was getting very frustrated, very annoyed, and very hungry. And then it happened—God pointed out to me a young man on the bus and told me to talk to him. But this time, perhaps because of my frustration and annoyance over the slow progress of the bus, the voice was not really clear and I had no idea what God wanted me to say to the young man whom I had never seen before and knew nothing about. And besides that, I was with a friend and we had our hearts set on eating lunch at the buffet and I did not want to get involved in a conversation that might cost us the opportunity to eat at the buffet.
Each time the bus stopped, I hoped that the young man would get off, so I could put him out of my mind, but he didn’t. The buffet was at the end of the bus route, and God seemed to be getting a little insistent, so I decided that if the young man did not get off the bus till the end of the line, I would talk to him.
The bus came to the end of the line and the young man and I and my friend got off the bus. And then it really happened—God told me very firmly to talk to the young man, but alas! He did not tell me what to say to him. I walked up to him and, just like that, God told me exactly what to say to him. The words were, “I want to show you something.” I spoke those exact words to the young man, and he replied, “OK.” God then told me to take him to a building three blocks away and show him the second floor that I had leased in the oldest surviving building in downtown. The young man walked with me to the building and I took him upstairs and showed him the second floor.
To make a long story short, this young man returned to that building everyday for the next six days, and on the seventh day he accepted Christ as his savior and became my closest friend.
Obeying the voice of God cost me my lunch at the buffet, but the reward was more than worth it. As for my hungry friend on the bus, he and I subsequently served God together side by side for the next six years.