A pea comb looks like a row of peas, while a single comb is like teeth on a comb.
As chicks, it can be very hard to tell between the two until they develop. Usually the single comb will look like a mountain ridge, while the pea comb will look like the hills.
As chicks, the pea comb base is wider than the single comb base, which can help when choosing EE's....look for comb bases that are wider.
Here are the comb types on adult birds.