The acid test is used to test the acidity of the wine.
Yeast need an acid environment to ferment the sugar.
Next to that a wine with no acid just does not taste right, the wine would be out of balance.
And last acid works as a conservative and will protect the wine
from some bacteria.
So acid is an important part of the wine.
If you do not have access to an acid testing kit like this one:
http://wijnmaker.blogspot.com/2008/11/zuurmeting-01-measuring-acidity-01.html
You could go by taste.
Normally any juice you buy will be in balance. There will be
acid in present.
But if you are going to dillute the juice the acid will also be dilluted
and the juice will be off balalnce.
You can bring the level of the acidity up again by adding some lemon juice. I did a story on that a few months ago:
http://wijnmaker.blogspot.com/2008/03/citroensap-lemon-juice.html
Do not overdoo it.
A high acid wine with a lot of alcohol will be off balance also, it will taste like jet fuel.
You can add some acid and correct the balance when the wine has finished.
Even better is to buy an acid testing kit over the internet if there is no supplier neerby.
Luc