By walking around and observing what is growing for “weeds”and how hard the soil is, is a way that I can tell how healthy a yard is. Typically, I want to sample the soil and have it analyzed at a soil testing facility like UMass Amherst, so a baseline of what exists (ie. pH, fertility, organic matter, heavy metal contaminants) is established.
“Weeds” are indicators of what is going on in the soil. Some weeds indicate that the soil is very compact. Others show that the soil is low in a particular nutrient like nitrogen or calcium.
Finding out how the owner wants to use their yard gives me the information I need to adjust or enhance its quality to fulfill these requirements. An analogy I always give is “Soil is to plants as blood is to human beings and animals.”