- Units - For example, if you know the answer should have units of force then an answer with units other than force can be eliminated.
- Limiting Cases - For example, if you know the answer to a problem is in the limit that the radius goes to infinity then all answers that do not obey that limit can be eliminated.
- Symmetry - For example, if you know that the answer must be an even function then all answers that are odd functions can be eliminated.
- Correct Sign - For example, if you know that the energy should be a negative quantity, then all answers with positive energy can be eliminated.
- Variable Dependence - For example, if you know the answer must depend on the radius and length, then all answers that do not can be eliminated.
- Plugging in Numbers - For example, it is usually easier to test if a vector is an eigenvector of a matrix than it is to solve for the eigenvectors of a matrix.
The techniques above can help you check your answers, find the right answers and/or eliminate wrong answers. It might be a good idea to keep these techniques on your mind while working through practice problems. Also, while reviewing your performance on practice problems, it might be beneficial to take another look and see if you could have used any of the above techniques to improve your performance. I would pay special attention to "limiting cases" and "symmetry" as they are perhaps some of the most powerful tools to have in your physics tool chest.
I find the techniques above are most beneficial on problems that I find difficult to solve. Instead of leaving the problems blank, I can usually eliminate a few answers very quickly and then make and educated or even random guess from the remaining answers. Feel free to check my math, but if you eliminate 1, 2, or 3 of the answer choices then that statistically corresponds respectively to 1/16, 1/6, or 3/8 points added to your raw score (assuming you guess randomly from the remaining answers).
It is worth the cautionary remark to mention that the techniques highlighted above will not benefit you on all the Physics GRE problems. Often times this approach can be more time consuming than simply solving the problem without even looking at the answers. Obviously, the goal is not to solve every problem on the Physics GRE in two different ways, but rather to find the approach that equates to the best utilization of your time. However, if you practice the techniques above, then they will be there for you if you need them on the Physics GRE and throughout your physics career.