I don't use coffee grounds to acidify soil. I just put them on my compost pile. I've heard that most of the acid is leached from the grounds as coffee is brewed. For myself, unused ground coffee is too expensive to use in the garden. You might have better luck if you use both the used coffee grounds and
any leftover coffee. The leftover coffee might contain more acid.
If you're curious, you could do a quick test with some litmus paper. You can even make litmus paper at home pretty easily.