[header] How to tell the difference between a cockatiel and a cockatoo [title] Look at size. [step] First, consider the size of the birds. Cockatiels are generally much smaller than cockatoos. 
Cockatoos do range in size, as they come in a variety of types, but can grow up to 26 inches tall. Cocktails, conversely, are usually only 13 to 14 inches tall.