While there is some basic information on the web, the best might be to visit a lien auction.

When you go visit a REO property if its been cleaned out well, try and see if you can get the info for the company that did the cleanup. 

Like others said visit as many REO's as you can to get a feel for them and what to generally expect.

Or fly to come visit a market?

A couple for you for my own edification which I don't have a great solution for thinking about it.Suppose I am an out-of-state real estate investor.And suppose there is some probability of any particular A class neighborhood turning into D class in the next few years.Suppose also I go and visit a target neighborhood/zip code and underwrite that the place is good to invest in.

In washington state they would also need to call it an estimate visit as doing a home inspection without a license is frowned upon by the state.  

@jimbryant I have searched in Peoria, went to visit a couple of times, however don’t know what areas are best.  

(If you wholesale a deal, if you rent your own property, If you place a renter for another investor, etc.)You may need to visit a number of brokers, and that's ok. 

I consider making an onsite visit a critical portion of this- get to know the provider, the neighborhoods, their rehabs and their ethics for operating.

Go visit a few dozen.Then look for the area you are comfortable living/renting in.If you identify an opportunity, compare it to recent sales.