Scoop some soil into a container. Then, add a half-cup of vinegar. If the soil bubbles or fizzes, it's alkaline.
If there's no reaction, scoop a fresh soil sample into a second container. Add a half-cup of water and mix. Then, add a half-cup of baking soda. If the soil bubbles or fizzes the soil is highly acidic.
Amend your soil with wood ash or lime, if it's acidic. Amend your soil with sulfur or pine needles, if it's alkaline.