Being able to taste a beer and dinstinguish what ingredients were used and in what proportions requires a very developed sense of taste and smell. Folk might think they have the prerequistes but this takes time, experience and effort, effort being the main one I think. Popping a beer, indentifying what aromas they smell. Tasting it and identifing the flavours. If you can taste a beer and name 10 aromas and 10 flavours then you're on your way to cloning a beer, and in the process becoming a beer connoisseur youself. But as I said, you need to do this often, with every new beer you try. I don't.
At the same time, you need experience using the various ingredients used in brewing, the malts, the hops etc. This also takes time and experience.
This is why I like clone beer recipe books...they help you understand the ingredients and what is necessary to produce a particular beer that you're familiar with. You can then use this knowledge to produce something similar, at least on paper!
I've heard of people formulating a particularly good recipe of a beer and then spending years tweaking various aspects of it, looking for the perfect combination of ingredients and process. This is the art of brewing. Long live the brewer who can perfect his recipe, but his passion is over when he does...