For your rental property, I suggest speaking with your tax professional prior to attempting to sell. They can explain any additional tax consequences or benefits.
In general, real estate professional(s) can come to the home and gather information to perform a Comparable Market Analysis with emphasis on recent sale activity for similar homes. Having more than one agent perform this will give you a range of value for the property. Alternately, you can hire a local licensed appraiser and pay for a not for refinance appraisal of the property. This will give you their estimate of the properties value. Neither method will take into consideration how much you owe on the mortgage.
If you are not currently under contract or obligation with another agent, I can give you the names of a couple of agents. You can contact me through Trulia by clicking on my photo.