The only answer is to have someone who you can trust eyeball the property for you. You can trust what the seller says, but you always have to VERIFY.
Our real estate agency is based on doing everything that out of state investors need to do to invest in our market. We select a property and prepare a rough estimate of rehab cost and verifiy that with contractors once property is under contract during the inspection period. We then oversee the rehab process and then manage the property for the investor.
Typically, people come visit us and we show them similar deals and jobs and give a tour of Memphis, but there are other investors that I have never met and have sold them a lot of properties they have never seen.
Obviously, it is easy to get taken advantage of at a distance. We have many people call us once they have already bought the property and very often they paid twice as much as they should have and it is in a bad neighborhood.