This is pretty interesting. In the Old Testament God spoke mainly through the prophets, but he also used dreams, visions, angels, miraculous signs, and other ways. Most of the time each event was totally unique and unrepeated. So if you’re waiting to hear God’s voice in a burning bush (like Moses) or see fire flare from a rock and consume meat and bread (like Gideon), or hear a donkey speak (remember who?) it’s not going to happen again.
When Moses, Gideon, David, Noah, Abraham, Joseph and others heard from God they all were absolutely positive that it was God and they were absolutely sure of what He told them. Actually you can have a personal experience like those, but it will be unique to you. You will know it is God speaking (maybe through prayer, circumstance, another person, your Bible) and you will understand.
In the New Testament God spoke through Jesus Christ. Read the whole New Testament if you want to know what God said.
Today God speaks through His Holy Word, i.e. the Bible, and He also speaks through the Holy Spirit. If you’re looking for truth you won’t ‘discover’ it, instead God will ‘reveal’ it to you through His Word or through the Holy Spirit.