You don't. And then you do. And so on.
The simple answer to "how do you know you're ready?" is: when you both think you are.
You will never know you are, and chances are you are actually not aware to all the implications. I have never met a couple that didn't go through this phase.
For what it worth, my two cents are: take it slowly, keep communicating and be sensitive.
And be ready for the roller coaster ride of your life, at least when you start.