When someone asks you at work or out and about, "I know that's what you believe, but how do you know it is true?," they are asking a question easier to answer if you can see the whole range of options. Most of us Christ followers would say, "well, God has changed my life," or "I had a life changing experience." That is fine and well, it is true. But if a person has not had that yet, what would you say to them next if they still ask, and they have not had your experience?
There are several ways to go. When you care about your friend, and love them, you want them to know God personally through faith in Jesus and to trust His Word to them in the gospel message.
One option is to bring in testimony of others, to show that you are not the only one with that experience. But this is again still the area of experience.
You can talk about self-evident truths, such as that there are other minds out there, even though you have not seen them in action in person, such as you believe rightly that your neighbor has a brain. That is a given. You can say the One True God is also self-evident, and be reasonable in saying that. You can talk of conscience and sin as pointers in this way too.
You can also call this self-evident truth collection (God, sin, etc) first level knowledge. It is had without having to process or ergo/argue to a conclusion. It is just acknowledged. Those who deny it are covering up something. While this sounds like a bad route initially perhaps, it is actually a very strong position (one of ways we know in Scripture, it is, Romans 1). Other self-evident truths have to be appealed to by any argument/view/position. So you can too. We can get into arbitrating them another time, another post.
The main thing in this post is to not fear self-evident truth appeals. You can share your faith and defend the faith with them. Blessings.