How do you deal with Flaw and Necessary Assumption questions?

Here's a start:

The flaw of any argument is the fact that the argument has assumed some information. In order to succeed, an argument must move smoothly, building from one point to the next without gap or interruption. When an argument fails to provide sufficient evidence for its conclusion - when it assumes that some important piece of evidence is true rather than demonstrating that it's true - that argument has failed.

Often, an argument will indicate its flaw on the basis of a shift in language: If an argument begins by saying that Mechanical Engineering majors are astonishingly physically attractive as a group, and that therefore, they must be a successful dating population, then that argument is flawed. The flaw is that it has failed to consider that the physical attractiveness of a group may not indicate its dating prowess. One necessary assumption of this argument is the assumption that they physical attractiveness of a group has some relationship to that group's success in securing dates.

So, identify the shift in language, and you'll have found the shift in logic. That shift is where the assumption lives, and that assumed evidence is the flaw of the argument.