One can differentiate between pentan-2-one and pentan-3-one by iodoform test.

Pentan-2-one have a –CO-CH₃ group and therefore, forms a yellow precipitate of Iodoform. Pentan-2-one gives positive iodoform test, whereas, and pentan-3-one does not give positive iodoform test because they don’t have a –CO-CH₃ group.