The white head and underbody with barely any visible markings it obvious here, also note the shawl effect on the nape of this bird which then bleeds partly onto the birds mantle. The small beady eye is evident and a small dark mask is also present encircling they eye. The grey mantle feathers with dark centres are obvious in this photo and this feature is particularly useful to look out for when trying to pick out a first-winter Caspian Gull. Finally the long and obvious parallel sided bill is evident in this image; compare this to the bill on the (presumed 2nd winter?) Herring Gull behind the Caspian.
You can see that in flight it shows a white head with small dark eye, the shawl effect on the nape can be seen quite well in this high quality image. The axillaries look greyish/white, but this feature is very variable in Caspian Gull and some can show brownish axillaries.
Compare to the above image and it can be seen how a birds axillaries can change in the field depending upon lighting conditions. But note the very white head, with black beady eye set far forward in the head and the shawl like pattern on the birds neck extending down onto top of mantle.
You can clearly see the broad and solidly black terminal band on the tail contrasting with the fairly sparsely marked rump/uppertail coverts. Nice contrast between the secondaries and the much paler coverts which creates clear cut black and brown areas on the birds upperwing. Also you can see the grey mantle and scapular feathers with dark centres creating a fairly distinctive appearance. Overall first-winter Caspian Gulls are usually four coloured; white, brown, black, and grey. This is particularly evident when a bird is on the deck, compare this to Herring Gulls which can often look less well defined and don't usually show such a sharp contrast throughout the whole plumage at this age.