1. Creation - It is so easy to take nature for granted. Especially when we have a selfish focus. But take a day (or night) and go outside and spend some time soaking in the beauty of your surroundings. My personal favorites are stars, wind and water (ok, and roses, I'll admit it). No matter what you believe about how nature came to it's current state, It's hard not to at least have a sense of "more-ness" behind it, like there is somehow more meaning to what you're experiencing than simply the arrangement of molecules and energy. Not proof, but a sign.

2. Conscience - This has been a big point in my own life. I went through a time where I had stopped listening to my conscience, and found myself doubting God's existence. Over spring break, I read The Barbarian Way by Erwin Raphael McManus. In the book, Erwin McManus talks about a conversation he had with his son. His son had gotten into a fight at summer camp. He asked his son if he heard any "voices" telling him what he ought to do. His son said "Yes, I ought to forgive the boy I got in a fight with." He then asked him if he could identify the voice, and his son said "Yes, it is God." With that story he suggested that our conscience is the voice of God speaking to us. Every time we listen to our conscience we are tunning our hearts to hear the voice of God, and every time we ignore our conscience we are hardening our hearts to hearing the voice of God. If we continue to ignore out conscience our hearts will harden and it will become harder and harder to hear God's voice, to the point where we will not only deny hearing it, but also deny ever having heard it at all. After reading that book, I decided to start listening to my conscience, and I found that God started speaking to me again. Voices in my head are hardly proof, but again, a sign.

3. Law - If you take the time to read the ten commandments, you'll realize that you haven't followed all of them. If you're concerned about being a good person that might bother you. That's the point. The Law points to the fact that there is a standard. Again, not proof, but it a pointer. A sign.