I just tested one with my pH tester pen, and the line is still purple (it would change to yellow if the tested item were acidic). So in this case - I don't know the brand of the paper towel, sorry - it's acid-free, but not necessarily lignin-free, which is also important.
It looks like there are lignin-testing pens on the market, but mostly for use by library preservation/conservation groups. The only reference I've found as to where to buy one - and the only scrapbooking-related reference I see to it - go to books.google.com, search for "lignin testing pen" (without the quote marks), and the first result should be a page from a 2004 issue of Creating Keepsakes that talks about acid and lignin testing pens.
What you really want to ask is -- are they 'Achivally Safe'. As stated previously, just b/c it's not acid-free, doesn't mean it's safe. It can be safe for use with photographs. but still not stand up to time.
So, if anything yellows, cracks, becomes brittle over time, then it's something to avoid unless it's for cards or something you don't care about longevity (and even that's relative).
My opinion is that paper towel would not be a good choice for the long term.