I would not invest in any real estate market where I had not spent a significant time in that market, talking to just about everyone I could. Interested in a specific state? Do some preliminary research on-line to see if rents and prices fit your criteria in the cities that interest you. How diverse and strong is the local economy? Who are the major employers and are they growing or shrinking? Are there investment opportunities near medical centers or universities? Search property management companies in the various cities and call them for information. Then pick the most likely city and take your next vacation there. Drive around neighborhoods that interest you. Call on every for rent sign and a representative sample of for sale signs and ask lots of questions. If there is an active investor's association in that city, get on the mailing list and try to attend a couple of meetings.
Once you think you have found an area that meets your criteria, get referrals for people that work specifically with investors. You need an agent, a reliable title company, property management, and some other other folks, depending on how hands on you want to be from a distance.
Research in the internet is helpful, but there is NO substitute for first hand, "boots on the ground" knowledge.