The best thing you can possibly do is assess the data used for the value yourself. Many times I meet with sellers and the agent before me did not show comparable or give a detailed explanation of why they recommended a certain price. What makes sense to you based on the comparable properties?
It should be backed up with data and easy for you to follow. If not be wary of that agent.
Secondly, as pointed out in previous responses, be careful of an agent "buying your listing". Meaning the agent over inflates a value so that you hire them and then beat you down on price after listing it.
Fair market value is not an exact science and in the best of circumstances you will get a 3-10% range. There are times when value is very hard to pinpoint with accuracy because the property may not have comparable properties. In that case I will tell the seller that but come up with alternative ways to price the property.
Following are some blog posts you may find helpful.
Understanding fair market value
Is Agent Buying Your Listing?
Price to Sell or Pay the Price Later