Tilth is a purely physical, structural quality of soil, so start by ruling out the tests that do not look at soil structure. A chemical analysis reports nutrient levels and pH, which says nothing about how crumbly or workable the soil is. A biological analysis reports microbial or earthworm activity, again unrelated to soil structure. An observational analysis is too subjective and not a standard measurement technique. What is left is aggregate analysis, which studies the size distribution and stability of soil aggregates through sieving, and this directly reflects soil tilth. \[\boxed{\text{Aggregate analysis}}\]