The term “opening your third eye” is a bit of misnomer because your third eye is already open, as are all of your chakras. If they weren’t. you’d be dead, because your life force flows through them every moment of your life. While there can be emotional or physical blocks that hinder the flow of energy, they are never closed.
What you are doing is developing your third eye so that more power can flow through it unhindered. Think of it like developing a muscle. The more you use it, the stronger it gets.
How will you know if you are succeeding? Your intuition will skyrocket, you will begin to have psychic flashes, and you will be able to project your power through it to command.
Edit to add: By command, I mean, you can literally use it to command lesser beings. In one book of qi gong I read, a student tested his third eye by projecting his power through it upon an ant and commanded the ant not to move and it literally froze in place, its antennae dancing around in panic.