I'm sure you've heard of Zillow.com, which allows you to see the approximate values of homes in certain neighborhoods. If you are looking for a home in or near a major metropolitan area, then you should be able to look it up on Zillow.
One HUGE caveat about Zillow is that even with its best estimates, the actual market value of the house - that is, what a buyer would actually be willing to pay - may differ by as much as 5-15% in either direction. When you look at a particular house on Zillow, it will present you with a specific number. However, if you dig a little deeper, you will find a "Value Range" that can be quite large.
Another thing that Zillow cannot do is actually look at the house to see what it's like. It bases its estimates on historical sales data and trends in the different neighborhoods. It doesn't know whether you've upgraded the appliances, laid down new carpet, or remodeled the bathroom.
The appraised value of a home (for tax purposes) is meaningless, especially if you're in California. But if you're interested, this information is part of the public record. You might be able to find this information on the internet. But this is going to give you little or no guidance about how to value a property.
With regard to the length of time a house has been on the market, your real estate agent will be able to give you this information very easily. You can often find this information using a real estate search engine. However, your agent will also be able to tell you if the sellers are being tricky about the days on market. Some sellers, after their house has gone unsold for a long time, will take their house off the market and then list it again a few weeks later. This resets the "days on market odometer" on the internet search engines, so an independent search might fool you into thinking that a house is newly on the market.
Perhaps the best way to figure out the fair market value of a house is to look for comparable recent sales in the neighborhood. Look at number of rooms, square footage, etc. You can calculate the cost per square foot, but this method, like Zillow's method, doesn't take into account what's been done with the house. An unremodeled house that sells for $300 per square foot might not be as good a deal as a renovated house that sells for $350 per square foot.
