The key features, which vary from species to species, are shape of mouth, lateral line scale count, relative position of dorsal to pelvic fins, fin ray counts, ventral keel, position of eye relative to snout, and to a much lesser degree, colouration (easily corrupted by photography). Some hybrids are easy to spot eg roach x bream but others, like this one, are more difficult. I have hundreds of pics of hybrids and also the parent species so have something to compare with, and also have several scientific papers on hybrids which have base-line figures for scale counts etc. Even the same type of hybrids vary a lot in appearance, and a hybrid with parents of male roach and female rudd won't necessarily be the same as one with parents of female roach and male rudd even from the same water - there is a view that maternal characteristics tend to dominate.
I catch hybrids regularly on the waters I fish, usually roach x bream but also rudd x bream, roach x rudd, occasionally silver bream x roach and silver bream x rudd, and have also had various crucian/carp/goldfish hybrids.
I referred this fish to another very experienced angler who has a similar interest in hybrids and he is also of the opinion that this is a roach x rudd. Bear in mind it's only an opinion and it would take a DNA test to be much more certain!
I investigated hybrids a fair bit for my Big Roach book, and that has a section in it with scale counts and how to distinguish roach hybrids, as well as some interesting pics of hybrids ( three of roach x chub).