Past Guides can't be accessed in person. A current guide can be. A current guide can perform miracles, those miracles prove a power vested in him from God that also proves God exists and realizing the magical nature of guide, you can that move on to witness his light reality/sky reality, and then connect to the guides of the past as well which are lights in the sky (Ahlulbayt) in this world.
Now, you will say, where is this current guide? Good question, we are living in hard times, in which the leader and guide is not in public. But if you believe miracles are evidence and part of the path to be guided and attain certainty, and promise you won't accuse the guide of being an evil sorcerer or one possessed/deceived by demons giving him such power than I will suggest praying the following with belief God CAN respond IF he exists.
I'm not asking you to believe in God or his Guide before the miracles, just, pray to God in a way that if he exists, he would respond and guide you with a promise that you will accept miracles.
The Quran says "And the disbelievers say why a sign not revealed regarding him, you are only a warner and for every people, there is a guide!" (13:7)
A believer when he prays...
"Guide us the straight path" (1:6) believing what Quran has described about the Masters of the path and the Guides of it, will be guided to the Guides eventually.
It's only those who take all the Quran talks about including miracles playfully never preparing their souls to accept miracles and never reflecting over the issue of sorcery vs miracles and power from God discussion in Quran and reciting Quran heedlessly, that God out of his compassion, knowing their souls are corrupt and would not benefit from the signs, that would be deprived of reciters of Quran from miracles by the hand of the Guide.
No doubt, if you understand what Quran has described as a proof for himself, and the many reality of that proof and guide, even without believing in God and guide, you can see if God exists, this would be the best way to guide his creation and an absolute proof of himself and guidance.
In this case, just pray to God whether you know he exists or not, this is all God wants, humbleness and a promise not to reject his proofs when they are proven but rather be reasonable and humble to them.