The Slender-billed Gull is most easily identified by its distinctive profile, with a long, sloping forehead and a long, slightly drooping beak.
It has long, blackish-red legs, a dark red beak (which can look black at a distiance), and yellowish-white eyes, with a red eye ring.
Juveniles have greyish markings on the head, and immature birds have darker markings on the wings and more pink-orange legs than the adults.
Outside the breeding season, the Slender-billed Gull sometimes has a small, dusky spot on the side of the head (clearly visible on the 1st & 3rd photo).

In the company of Heuglin's Gulls and a Greater Crested Tern (yellow bill).

Mughsayl Beach area, Dhofar Region, Oman, 3 February 2017