700 level questions in the GMAT REVIEW book

There aren't all that many really high-level questions in the Official Guides, unfortunately, though as IJR says the questions generally get harder near the end of each section. Most of the questions in the OG are quite old now, and were written by a different company than the one currently developing test questions. The top level of the test has gotten harder in the last few years, and you'll get a much better impression of how hard questions can be if you're in the 700+ range by taking the GMATPrep tests (which you can download for free from the official site).