I'm a realtor in Wyoming. To find property value via the MLS for myself and my clients:
Find comparable properties that are actively for sale. You'll want the price to be just below the lowest comparable.
Find comparable properties that have sold within the last 6 months. You'll want to be just above those.
Find comparable properties that have expired (been for sale but not found a buyer). These will not have sold because they're priced too high. Keep these in mind as they'll be an indicator.
These three comp sets will indicate market value. Multiply by .8 and you'll know what 80% of market value is. (Usually a range)
Zillow has gotten much better in the last few years for some markets but still unreliable. Use the numbers as a guide but not as conclusive.
Tax assessment may also be an indicator but be careful because the municipality may use a convoluted formula involving multiples of mils to determine the value and it often has little relationship to real market value as it serves a different master than the free market.
Gosh, that's a lot of words! PM me to discuss in more detail if needed.
JK