Visual Quality Indicators for Soil and Plants
There are numerous visual indicators for various type of plants that determine plant health. For example the rigidity of the leaves can help you determine that enough water is supplied to the plant (1). Now you can also determine the leaf shape and detect for presence of disease (2), pests (3), and plant health in general (4). This research however mentions no correlation between lamina shape and area so you would need wiser methods for your corrlelation methodology(5). Moreover, you can look at the fruit shape/sizes to determine if plants have been supplied enough nutrients(6). Another study which is worth mentioning is using chlorophyl to determine plant fitness but this cant be done insitu and you would need lab analysis (7)
All these are visual indicators for the plants itself. As far as soil is concerned is it more challenging because you can make soil and compost using various materials and methods so there is no general approach that can be used to determine the quality. So if soils is your primary concern then as @PolypipeWrangler mentioned looking at the plant fitness can help you determine soil quality.