Hi Kelly —
I don’t know of any simple test for lignin. Your best bet is to ordermaterial that’s specifically described as both “acid-free” and “lignin-free” from reputable suppliers.
I rely upon Gaylord in the course of my contractual curatorial and archival work for museums and historic sites, and have been very happy with their products and service., but they’re not the only supplier by any means.
If you do by products that are acid-free but not lignin-free in the interest of economy, it would be a good idea to set samples aside and to test them on a regular basis — about once a month for a year, and once a year…forever, or until they give an acidic reading with a test pen.
Buffered paper products cost a bit more than simply “acid free” products (but lots less than lignin-free stuff), and they will retain their non-acidic pH for a longer time…but not forever. Periodic testing of samples would still be prudent.