Daniel, I'm afraid that you are going to have to provide a bit more information about what it is you want to know. If you are talking about an employer-sponsored retirement plan, you should contact your employer's human resources department. They should be able to give you the information you are looking for. If it's something else, please post another question with more information. (Note: please don't post any personal information such as account numbers, etc. I've seen a few people do that here.)
What plan? Where? You should be receiving periodic statements that provide the value of your assets in the plan.
Please give us more information on what type of plan you are talking about. If it stems from a past or current employer, give them a call and ask them for information. Also, make sure the plan has your current address.