Weeds That Say Your Soil Is Sweet
Campion, field peppergrass, nodding thistle, salad burnet, scarlet pimpernel, and stinkweed all indicate a "sweet" alkaline soil (pH higher than 7.0).
Ornamentals that do well in alkaline soil include lilacs, Persian candytuft, dianthus, baby's breath, helianthemum, dame's rocket, lavender, and mountain pinks. Some edibles also tolerate soil that's a little on the sweet side, including asparagus, broccoli, beets, muskmelons, lettuce, onions, and spinach.
If you want to lower the pH of your alkaline soil, add peat moss or elemental sulfur at a rate suggested by soil test results. Or, again, simply add compost regularly to bring the pH closer to neutral.