The Al Bhed can be identified by their blonde hair and green eyes with swirly pupils, which are common physical traits exhibited by the race regardless of one's skin tone, which can range from medium/tan (i.e: Rin) to light/fair (i.e: Rikku). While having blonde hair might not be a feature solely distinctive to the Al Bhed, their green eyes with swirly pupils are certainly the race's most distinguishable and unique physical trait.
The importance of their swirly eyes is highlighted when Auron examined Rikku after Yuna asked him that she would like Rikku to join the group. At first, Rikku tried to hide her face. She even closed her eyes when Auron asked her to look at him because Rikku wanted to conceal the fact that she is Al Bhed. (Generally, the Al Bhed do not follow the Yevon religion, so they do not typically join in on a summoner's pilgrimage.)
A person of Al Bhed descent can also be easily identifiable through their fashion choice. According to Nakashima Fumi, the sub-character chief designer of FFX, since the Al Bhed are a machine-oriented society, they are purposely designed to appear different and eccentric from other inhabitants of Spira.1
Most Al Bhed are seen wearing goggles or gas masks, perhaps to hide their eyes, or, for those that live at Home on Bikanel Desert, to protect themselves from the desert sand. The typical menswear usually range from fitted sleeveless tops or fitted scuba-like long-sleeves tops made out of a mix of leather and other fabrics, along with long pants either laced-up or tugged into the boots. Clothing for female tend to be more revealing, with some sporting bikini-like outfits and many opting to wear shorts with thigh-high boots. For both genders, their outfits can sometimes be seen being adorned with belts that look like safety straps and saddles wore on top of the pants/shorts. The Al Bhed also tend to like to cover up their arms with many choosing to wear arm sleeves and gloves. Children are usually spotted wearing full body-suits and are the most covered-up.
1. Developers Interviews; Bonus DVD -- FFX International/PAL versions