The acids in wines can spell trouble for some people with sensitive stomachs, but unlike alcohol content, there's no indication on the wine label of the contents' acidity. With a little bit of basic knowledge you can guess on the acid content, though. Typically, wine with high acid content are white wines, sweet wines, and wine made from grapes grown in cooler climates.

The The Miami Herald gives a more in-depth explanation:

All grapes produce fruit acids - tartaric and malic - as they develop. In cool climates, acids are higher; in warm climates, acids are lower. After fermentation, winemakers put many white wines and most red wines through a second process called called malolactic fermentation to produce the rich, round wines Americans love. This converts the sharp malic acids into softer lactic acids similar to those in milk.

When the wines are finished, their acids are measured in two ways - total acid (TA) and pH. Here's how they work:

• TA or Total Acid (the higher the TA, the higher the acid):

- 1.0 grams per liter (g/l) is too high; the wine is tart and sour.

- 0.4 percent g/l is too low; the wine is flat and bland.

A typical California wine is around 0.6 percent g/l.

• PH, another way of measuring acid, works in reverse - the lower the pH, the higher the acid:

- A pH of 4.0 is too high, meaning too little acid; the wine is likely to spoil.

- A pH of 3.0 is too low; the wine is tart.

A typical California white wine has a pH of about 3.4; a typical California red has a pH of about 3.6.

The article also gives a few specific examples of high acid/low acid wines, if you are interested.