I've heard from others that litmus paper gets stained from the sauce and is hard to read the color. Can't say from personal experience though-
There are other pH testers available for $20 and up. Once again, you will get what you pay for, but it's a start. Also, make sure to order some buffer solution if required for the tester. It's not that expensive.
If you order an inexpensive tester, keep in mind that it will not be as accurate as the more expensive models and allow yourself a margin of error when taking readings and deciding on the final pH of your product.