You can’t know what the first question in your viva be will be until it’s asked.
You can’t know how long your viva will be until your examiners say, “OK, we’re done!”
You can’t know how you will respond to a tricky question until you experience it.
You can’t know in advance just what you’ll need to correct after the viva.
There’s a lot you can’t know before you get to the viva and experience it. That’s just the way it is.
But you can know that you’ve done enough to get you there. You know you’ve done enough to succeed. You know who your examiners are in advance, and can know all about their research, if that helps. You can know what to expect from the viva by reading regulations and listening to stories of viva experiences.
When you stop and think, there’s a lot you can know before you get to your viva.