What does 6% acidity taste like?
In wine-making, at least, acidity is a function of three different types of acid.  Because those types of acid matter to the final product, wine making supply stores and websites provide several inexpensive ways of determining acidity.  Two described here are an acid titration kit and a pH meter.  When making wine last year I used an acid titration kit and it couldn't have been simpler.  You take a sample of your liquid, the indicator solution, and a reactor solution.  Based on how much of the reactor you have to add to change the color of the original sample you can get your acidity in a percent.
In my experience with the wine we noticed the acidity in our grape juice (that was on its way to becoming wine) more by the burning sensation on our hands where we touched the grapes as we crushed them or in our mouths when we drank - our solution started out seriously acidic.  It was hard to taste a difference between too acidic and just right.
If you have a liquid recipe and want to get the acidity exactly right, consider using acid titration.