Just count the number of double bonds (=) in the structure. If it's odd, there's a chance that it'll be aromatic. If it's even, then it can't be aromatic. This molecule has one double bond, so we're good to go. Is it planar? In this example, the molecule is planar since 3 points define a plane. Do all atoms have an empty p-orbital? Yes -- each atom is sp-2 hybridized. There's an empty p-orbital on the cation carbon that is able to store electrons and p-orbitals overlap in the pi-bond! Therefore, it satisfies all the rules for an aromatic compound.