What is the dominant shape of a pea pod how do you know?

The dominant shape of a pea pod is Smooth.
To know this we simply need to see the shapes of the majority of pea pod around us and leave the rest to natural selection. The majority of them had smooth shapes.
This indicates that the Genetic that determine the smooth shape of pea pod is Dominant and exist in larger amount.

The dominant shape of a pea pod is smooth because that is the phenotype that showed up in F1. F1 seeds or plants are made with selective breeding and are created by having two different parent plants cross pollinating. F1 means first children and this child inherits traits from both parents. Since F1 inherited the best traits it is safe to assume smooth is dominant.