Classically, discretes are on a page that says something like: The following questions are not based on a descriptive passage and are independent of one another (paraphrased). But you're right, some passages have discretes: like you said, sure the passage is talking about chemical reactions, but the first question might be about the meaning of exothermic which you could have answered without reading the passage. Then the other questions usually get into the meat and potatoes of the passage.
The idea of skipping to the discretes might have made sense with the paper MCAT but I don't think that makes sense with the CBT. To begin with, switching pages is different and more time consuming especially because the aamc tests don't have page numbers clearly identified (instead you have to click next and previous). Also, if you read the threads on the Jan tests, no one said that the discretes were that easy anymore (many said the exact opposite). I would not waste time sifting around the exam: practice, get your speed honed and then go for it.
BTW, what's "RLs" ?