The attempt is submitted for marking when you click "Submit all and finish".  Once you have done that you cannot try again: each lab quiz can be submitted for marking only once. 

The attempt is submitted for marking when you click "Submit all and finish" but you can try again after that if you want to have a new attempt marked.

You need to click "Submit all and finish" if you want to save the attempt so that you can come back to work on it again later.

Click the Check button at the bottom of the question as soon as you've answered it.

Just go on to the next question and your answer from the current one will be automatically checked.

Click "Submit all and finish" at the end of the lab quiz

There's no way to find out if your answers are right until after the lab quiz has closed.