3 Answers
One cannot distinguish whether a plant is C3 or C4 by observing its leaves and other morphological features externally. Unlike C3 plants, the leaves of C4 plants have a special anatomy called Kranz anatomy and this difference can only be observed at the cellular level. For example, although wheat and maize are grasses, wheat is a C3 plant, while maize is a C4 plant.a
Plants which are adapted to dry climates follow the C4 pathway. While this knowledge can be helpful in telling a C4 or a C3 plant, external features hardly provide a clue for this. However, histological study of bundle sheath in the leaves can easily give clue about the plant type.
Usually plants growing in dry conditions use C4 pathways. It cannot be said conclusively if the plant is a C3 or C4 by looking at external appearance, some guess can be made by looking at the fleshy leaf structure.