Ok as promised here is the name of the company that manufactures the PH testing pen. Lineco, the item number is L533-0023. Their website is
www.lineco.com or you can use the link to purchase one
www.redimat.com/products/accessories/l533-0023-lineco-ph-testing-pen.html
It's not a definitive method but the people I work with tell me that all pre-90's documents that have been improperly stored should have a PH level that is really low indicating that the paper is acidic. I think a build sheet that has been sitting under the carpet, seat, gas tank or underlayment etc. fits the improperly stored scenario.
The guys I work with also stated that you need to look at the paper under a high powered jewelers loop and look at the texture and grain of the paper of known original documents. Todays paper has a lot of recycled content and looks different under a loop or microscope.
Also make sure you store your documents in acid free folders and proctective sheets in order to preserve them.
I am going to be talking with one of the conservsation and preservation companies today or tomorrow and I will ask them about any quick and easy method to determine a documents age.