litmus strips are better than nothing and will give an approximate reading. If I remember right, the strip changes color to indicate the pH range. Some sauces can make it difficult to see the real color.
Sawyer is right, some meters are not made for heated products.
One other method is to put a little ramekin or glass dish in the freezer ahead of time. When you're ready to test the sauce, put a teaspoon or so in the frozen dish. The sauce will chill down right away. Put several dishes in the freezer ahead of time if you think you might be doing multiple tests. It's faster than waiting for the dish and sauce to chill.