Because Hashem isn’t out to get us. He isn’t arbitrary and capricious. He isn’t going to give us tasks and then not give us a way of determining what those tasks are.
How do we know Hashem isn’t, k’ilu, looking down at us and saying, “Look at all those mitzvos Reuven is doing– look how diligent he is– but why isn’t he focusing on helping Shimon do more mitzvos?”
Yes, I think He is. Which is why you need to have a rav.
How do we know which rabbonim to follow
You choose one, in good faith. Hashem isn’t out to get us.