If still in the packaging, you can hopefully read the label to make sure. If it is truly acid-free, the packaging usually says so. Any paper packaging that doesn't say “acid free” probably does contain acids but how do you make sure? Also, how do you check paper that has already been removed from the packaging? How about something that was printed years ago? Was it printed on acid-free paper or do you need to copy it to modern acid-free paper?
Luckily, a pH Testing Pen can answer all those questions. Best of all, it only costs $3 to $6 or so.
To test the paper, use a pH Testing Pen to make a small dot or line on the paper.
The pen contains Bromcresol Green, a chemical ink that changes color when applied to paper. If it turns yellow, the paper has a high acid content; green indicates there is some acid present; and blue means it's acid-free. The blue dot remains on the surface to serve as a record of acidity at all times.
I have recently been using a pH Testing Pen from Light Impressions, a well-known retailer of archival handling products. The pen sells for $5.85 plus shipping and is available at http://www.lightimpressionsdirect.com/ph-testing-pen/album-accessories-scrapbooking-supplies/
I also have found a number of other pH testing pens available from other retailers. While I haven't used these myself, I can report the following are available for sale:
Amazon.com sells a pH Testing Pen manufactured by Lineco at http://www.amazon.com/pH-Testing-Pen-for-Paper/dp/B000KNJCSSYou can find others if you start at Google or most any other search engine. Make sure you look for pH testers for paper. Your search will probably find many other testers that test for acidity in water which is useless for most genealogy purposes.
The American Crafts pH Testing Pen looks identical to the Light Impressions pen but sells for $2.99 plus shipping at http://www.webyfl.com/phtestpen.aspx
For more information, see the following:
Comparison of pH Pens on the Market by J. Miller & E. McCrady: http://cool.conservation-us.org/byorg/abbey/ap/ap03/ap03-5/ap03-508.html