Hi Andrew(both of you!),
Would agree with what Fifebirder says above,but would add that adult birds can sometimes show an odd greyish-flesh leg colour in winter,and 1st-w birds etc also have pink legs(though a different pink to that shown by the 'ordinary' large gulls).
The call is on the
Calls of Eastern Vagrants CD,and is quite distinctive,though I'm not sure how useful this would be amongst hundreds of other gulls?
1st-w birds are also quite obvious,at least as large gulls of that age group go,and can be identified by a combination of structure,greater covert pattern(tends to have dark greater coverts with a pale tip,producing a pale 'wingbar'.Sometimes there is a second 'wingbar' formed by pale tips to the median coverts),head streaking(while even
michahellis Yellow-legged tends to have some streaking on the face etc in 1st-w,Caspian is much whiter-headed,with streaking on the nape contrasting with the white head),underwing(almost white,with no contrastingly dark area on the underwing coverts as in most other large gulls) etc.
There are lots of good gull websites out there,and I can recommend the literature suggested by Fifebirder above,and also an article in
Birding World back around Feb.00(?),not to forget Lars Jonsson's masterly dissertion on the ID of Yellow-legged and Caspian in
Alula back in '98.
Harry H