A good way to identify properties not yet on the market is to drive the neighborhood as often as possible (while you're in town), to look for Coming Soon & For Sale By Owner signs. These homes aren't necessarily listed yet on the internet or local Metrolist.
Your best bet though, is to find a good Realtor to advise you through the whole buying process. Good agents will work hard for you to identify homes that haven't yet been listed. There are multiple ways to find those homes, such as compiling a list of homes in the area that fit your requirements (simple lists should take only 24 hours to put together) and then mailing a letter to those homeowners, or better yet, knocking on the doors of your favorites. Other options might include your agent networking with other agents, contacting sellers who listed their home over the past few years but were unable to sell, or sending marketing e-flyers out to the real estate community informing them of your buying need.
Most importantly, you need an agent who will be hyper-vigilant in watching the market for new listings to pop-up. Good homes at good prices are selling extremely quickly, and so you want to hire someone that will put every new listing in front of you, as soon as they hit the market.
And, it is possible for an agent to help you mitigate the challenges of house hunting "from a distance" - while you're living out-of-state. If you have already hired an agent, ask them how. If not, talk to the people you know here in town, and ask them if they have a Realtor they know and trust. I'd be happy to help you as well. Make sure you hire someone who's business is real estate, and doesn't just dabble in real estate as a side job. Real estate transactions can be complicated. In Colorado, buyers' agents are paid by the seller so there's no reason why you shouldn't have a trusted adviser guide you through the process!
Kentwood Co Cherry Creek
44 Cook Street, 9th Floor
Denver, CO 80206