When I was younger and I asked "how do you know God is there / loves you / is listening?" the answers I got were vague and useless, no matter who I asked -- "oh, just have faith / you just have to believe." Essentially, I was told to ignore reason, ignore my mind, and just trust for no reason at all. And I would try, and I would scrape by, but I never felt safe or secure or fully able to believe in my own beliefs. My awareness of spirit was strong enough that I could never ignore it, but my mind was not in harmony -- half of me was left out.
Faith in Deity/Spirit is just like faith in any other being -- it comes from experience and research. Say you meet a person. You could choose to just 'have faith' that they are kind, trustworthy, honest, etc., but that faith would crumble the moment trouble came -- because you would have evidence AGAINST without evidence FOR. However, if you knew all their friends/family, learned their life story, spent hours talking to them, then your faith would be strong even if things looked really bad. It's not a bad thing to doubt and question -- that is the only way to build REAL faith that can withstand anything. I'm reminded of the parable of the person who built a house on sand versus the one who built a house on rock. Faith is the building -- but it is only as strong as the facts that you build it on. Even the most skilled builder can only do so much to compensate for a shaky foundation.
I once had an LJ friend whom I loved deeply and had a great deal of faith in. As time went by my mind picked up clue after hint that led me to believe that this person was lying to me, and my faith began to be chipped away by these things. I asked this friend for something solid which I could use to strengthen my faith, but ze refused. I tried to have faith without any proof, but I cannot lie to myself, and eventually I broke off contact with this person. I found it very hurtful that this person would not make any effort to help me -- instead, ze demanded unquestioning faith. If ze truly wanted me to have faith, why would ze deny me help when I confessed that I could not do it on my own?
The fact is, faith is not a matter of blindly accepting. It's a matter of seeking, being open, and then remembering all the experiences, all the knowledge, and putting it together to get a picture of life/love/spirit/God. Faith is not real if you copy it from others or take someone else's word for it, it's real if you build it yourself from the ground up. If you question everything and don't hold back out of fear. If I had never been willing to lose the false faith I had, I would never have built the very real faith that I now have.
Over the years I've studied various things to 'build my case'. One thing that helped a lot was reading up on NDEs and OBEs (near-death experiences and out-of-body experiences). My favorite book on that subject is Closer to the Light, written by a medical doctor and researcher who takes a (almost annoyingly) skeptical and non-assumptive view of NDEs -- ze proves that they are real experiences, not hallucinations or dreams, but leaves further interpretation up to the reader. Something survives bodily death -- I choose to believe that it is the true essence of a person, and that it survives forever.
Another thing is a book called The Secret Life of Plants. It's full of studies which satisfied my scientific, analytical side with their thoroughness / lack of presumption, and thrilled my spiritual side with what they revealed. The central focus, reflected in hundreds of varying studies, is that life is more than the sum of its parts -- that there is something more which has effect, which as of yet we cannot measure in any way. (I would call it 'spirit') It helps me to know that while science cannot prove what it is that exists beyond the measurable, it DOES show that there is more to life than the chemicals or measurable aspects that make up a body.
(((side note -- it bugs me when people assume that all spiritual people haven't put any thought into their beliefs and are 'illogical' when I know it is possible to have a very logical, rational belief in spirit/Deity. Ultimately it is an act of faith to believe that there is OR IS NOT spirit/Deity, as neither can be proven. I find it more logical to believe in spirit, or I would not believe.)))
So, essentially, there is proof that there is 'something' but it is still an act of faith to believe that that is God/dess, because there is no way to prove that. For me, it is the experiences I've had, the times I have felt zir/their presence and received messages from zir/them that make me believe. To me it is logical to believe in a personal deity(-ies) because I have experienced that in a way as real as anything else I've experienced. I have asked for signs, and received them. And now that I have found some solid foundation with which to strengthen my faith, it has grown by leaps and bounds. Now when I have an experience which many would class as 'supernatural' or 'crazy,' my mind understands that there are aspects of reality which cannot currently be explained by science, and I can believe in those experiences because of that. I'm able to trust in my spirit because my mind knows for sure that my spirit exists. My mind no longer automatically shuts out those experiences, because I have a way to understand them.
So my answer to younger-me's question "how do you know God is there / loves you / is listening?" is "because science tells me that there is more to life than the physical, and out of faith I believe that 'more' to be the spirits of beings, including Deities. My experiences with the personality and presence of God/dess show me that ze loves me and is listening, in the same way that my experiences with my friends show me that they love me and listen."
ETA: There is nothing wrong with 'blind' faith but it should not be an expectation, nor should it be held up as the ultimate kind of faith. Questioning faith is just as good, and for some people (like me) it is by far the best way.