kermitz

First of all, I couldn't find much after a quick research but if there is any articles about this topic I want to know. It might be a nonsensical question but how do scientists know if a fossil is deformed or not? I mean, if there is more than one fossil of that species, its understandable. But how do they decide if the fossil is a new species or just a major mutation of another existing species? And is there any chance of, for example, identifying a human species wrong because of a mutated skull? (I hope this isn't hard to understand because of my english)