I suspect you are thinking of something like Zillow's "Zestimates." For the most part, such automated estimates are fun, but have little use in determining the actual value of a property. The last time I checked, Zillow claimed their Zestimates were within 10 percent of the actual value of a property about 50 percent of the time, which means they are pretty worthless for making real world decisions.
I don't believe Trulia has a similar estimate system. You can ask a real estate agent for a comparative market analysis (CMA), which should give you a good idea about the market value of a property, but an appraisal can only be provided by a licensed appraiser.