How do I know which questions to spend time on vs. skip?

You're always making a judgment call, and you don't have to be perfect. If you're ever on the fence, save a problem for later with the mark function and do the solid ones first. Some people find it helpful to flip the question and ask "which questions should I not skip?" That makes skipping the default, so the only problems you attempt on your first pass are those with clear signs that they're among the easier ones and that you have a plan for solving. Either way, you're looking for signals that make your decision an educated one.

Four of the most useful signals are:

1) By type: if you learn to categorize questions by format and content area, you can use your past results to find your weakest types. Some students might choose to skip Geometry or Permutations, or in verbal, Strengthen questions.

2) By look and feel: this one takes training. You can learn to recognize at a glance the kind of dense quant word problem or challenging verbal subject matter that gives you trouble.

3) By length: for some people, longer passages (or shorter ones!) are meaningfully harder than others.

4) By plan: if you can quickly see a clear path to an answer in under 30 seconds, that's a good sign to continue. If you can't, it's a good sign to skip.