What is the method for gauging the success of a retirement plan when funds are deposited with varying amounts?

I get quarterly reports on my 457(b) plan. I almost always have gains, however, I am unsure on how to determine if the investment options are strong earners, or merely mediocre. What is the process for determining if the investment’s I’ve chosen are strong when the option amounts change due to varying bi-weekly contributions? How would I determine if the gains are good in the following example for one quarter?
Example:
Beginning Balance: ,011.33
Contributions / Transfers In: 0.00
Gain/(Loss) / Interest: 6.68
Ending Balance: ,518.01

Since starting in 10/02 I have contributed 93.00 and had gains of 25.01. However, the contribution amounts have varied greatly over this time period. So what is the proper gage on success? At 35 years old, my investments are diversified between high and moderate risk with some low risk mixed in.