Hi Sara, you may not get all of the information on Trulia and other sites that share listings but if you begin a relationship with a Realtor early on, you can receive listing data from the MLS for the property and criteria you are interested in. This includes a field that does show if there is melloroos. Of course the data is only as good as the people who put information into the database, the MLS is no exceptions, so, it's not always accurate, but i think it is highly reliable. You should always look for melloroos info on both your title report and NHD report which are provided during escrow.
If you aren't working with a Realtor yet, I'd be happy to set up the MLS Alert for you. It is automatic, you will have no obligation and I'm not pushy. You can contact me through my Trulia profile and if your on the fence about selecting someone to work with check out the feedback from my clients here and most recently on Zillow.