As I noted in a separate post, ARCO published a now out of print book containing 4 sample tests. The book is called "Mathematics: Advanced Test" by Morris Bramson. The lone review on Amazon.com asserts that the tests are remarkably similar to the actual gre. Having finally obtained this difficult-to-find book, i can confirm the truth of this statement. It is difficult to poinpoint what makes a "real GRE question", but it gives off a 'feel' different from say, REA (Best Test Preparation people) practice questions. Perhaps it's the fact that the problems which appear on an actual exam have been meticulously selected and scoured for any inconsistencies or ambiguities. Many are also selected because they are interesting problems, not merely requiring wrought calculation, rather, some sort of deeper understanding or creativity.
For these reasons I am surprised at how good Bramson's book is. The tests contain questions representative of the real thing both in scope and in difficulty (another possible shortcoming of REA's book, whose critics claim have some questions that are far too difficult). Each test contains 70 questions, and the choices occasionally range from A-G or A-H instead of the 5-choice system with which we have grown so familiar. Basically, there are some differences between the real test and the samples, but they are all secondary to the overwhelming resemblance the questions have to ETS-style ones.
I should also mention that the book contains no review of subject material, just some general information/advice regarding ETS's tests and how to take them. This section is useless, but sometimes ridiculous to the point of hilarity. For example, one of the pieces of advice is:
Keep Physically Fit. You cannot retain information well when you are uncomfortable, headachy, or tense. Physical health promotes mental efficiency. Guarding your health takes into account such factors as these:
a. Sufficient sleep
b. Daily exercise and recreation
c. Annual physical examination
d. A balanced diet
e. Avoidence of eyestrain
f. Mental health
It is a real shame that this book is so difficult to find, but will be well-worth it if you succeed in doing so.