There's really no short answers with gulls but very briefly this is a mich rather than a cach because (in no particular order):
1) what is visible of the greater and medians reveals a pattern of dark centres and irregularly shaped (notched) pale edges without any real pattern of contrasting pale tips forming pale wingbars. On cach the greater coverts in particular show a plainer or streakier pattern with neater pale fringes and diffuse pale tips which show up as quite a prominent pale wingbar in most birds. On swimming birds all but the inner third or so of the coverts are hidden by the flanks so this must be used rather carefully but there's enough visible in the first pic to rule out cach.
2) The bill is much heavier than that of most cach, has a prominent gonydeal point and and a rather steep culmen at the tip, the shape is somewhat reminiscent of a GBBG. On Casp the bill is finer, lacks such a prominent gonydeal point and has a more gently curved tip. To draw a rather extreme analogy (with Herring and LBBG viewed as comparable to a Common Gulls) cach looks more like a Slender-billed or Black-headed Gulls and mich more like a Med Gull.
3) relatively few cach are as worn as this in October, indeed such wear on the coverts and tertials at this date is very typical only of mich.
4) Most cach have much whiter head and body plumage than this bird and tend to have streaking concentrating on the hindneck and upper breast sides to create quite a striking shawl effect, cach rarely show such a concentration of streaking around the eye and the whole head often looks very white with a rather small dark eye looking somewhat isolated.
5) cach typically shows a more snouty appearance with a particularly long and shallow looking forehead that can again bring Slender-billed Gull to mind. The 'dignified' or 'proud' head structure of this bird is very typical of mich and most unlike the overwhelming majority of cach.
6) cach frequently shows less obvious dark markings in the scapulars so that the grey component in these feathers is even more obvious than on mich (though both show a contrast of grey scaps versus brown coverts and whitish head and underparts which tends to set them apart form Herring and LBBG of comparable age, the latter being more uniformly toned lack this tri-coloured pattern)
here's some links,
cach in October:
http://www.xs4all.nl/~calidris/caspianHL.htm
http://www.martinreid.com/cachp03.html
and November:
http://www.xs4all.nl/~calidris/Pontic3.htm
and December:
http://www.xs4all.nl/~calidris/PonticDec.htm
Some mich,
nice comparison with Herring in Jan (note more heavily worn plumage of mich on left and that it has replaced some coverts - very rare in Herring), top image:
http://oystman.tripod.com/italiangulls/id9.html
November:
http://www.magikcircle.com/birds/image.asp?title_id=459&show_thumbnails=False
A fresher plumaged bird in Aug, still with a few retained juvenile rear scapulars:
http://www.magikcircle.com/birds/image.asp?title_id=811&show_thumbnails=False
Another in November (unusually fresh for this date):
http://www.magikcircle.com/birds/image.asp?title_id=465&show_thumbnails=False
October (rather herring gull like head pattern but otherwise typical):
http://www.martinreid.com/michp04.html
Another October bird (note the replaced coverts - some cach are similar but very few Herring ever replace coverts or tertials in autumn)
http://www.martinreid.com/michp03.html
A view of the open wing would be totally conclusive but I can't see anything on this bird to point to cach and plenty that points to mich.
Unfortunately the best European gull website has been unavailable for some months now.
Spud