Hi kaylinamey!
Happy to address your question. You ask:
When should I know to find the alternate cause in order to weaken or find the flaw in the question?
You should
always be looking out for things like alternative causes that might weaken or be flaws in a given question. Of course, logic games are an exception since one doesn't encounter those types of questions on games. But it's generally always worthwhile to do on any reading comprehension and logical reasoning questions. Upon reading through a given stimulus in logical reasoning, you also might find that there is no conclusion. This is common for the must-be-true question type. In that case there isn't an argument to weaken. But you can still be ready to pre-phrase flaws or assumptions on each question, even if they turn out not to have any. Pre-phrasing these things is an important strategy that keep one engaged and also can help one get through answer choices more quickly.
You also mention a question about rattlesnakes, which I believe is question #22 from practice test 30. The conclusion of this stimulus comes at the outset: "The folktale that claims that a rattlesnake’s age can be determined from the number of sections in its rattle is false." Why does the author claim this? The author explains that its rattle
does contain indications of the number of times that its molted, but that pieces fall off because it is brittle, making it difficult to determine age.
After reading this stimulus, I was left with uncertainty about how molting refers to age. That uncertainty could potentially be because of a flaw or unstated premise, so it seems worth trying to pre-phrase something in light of it. Without keeping an eye out for flaws and assumptions, I might instead have inadvertently made some connection between age and molting myself--perhaps rattlesnakes molt annually, for example. Each notch in its rattle would then indicate a year. Or perhaps they molt monthly.
In looking at the answer choices, answer choice (A) might have been tempting if I hadn't been thinking about this ambiguity as a potential flaw, but nothing in the argument requires the rattle to indicate age according to years, rather than some other measure like months or weeks. Answer choice (E), however, states,
Rattlesnakes molt as often when food is scarce as they do when food is plentiful.
To test if this is a necessary assumption, we can apply the two steps of the Assumption Negation technique. First, we negate the statement:
Rattlesnakes [do not] molt as often when food is scarce as they do when food is plentiful.
Second, we ask if the argument falls apart with this negated statement put back in, in which case it is a necessary assumption. The argument would fall apart if we added this statement, since this would suggest that the number of times a rattlesnake molts isn't constant but rather varies and fluctuates based on their food source; if that's true, then the rattle can't be used to judge age. The Assumption Negation technique therefore confirms that (E) must be the correct answer.
