"When flying IFR, how can you find out about preferred routes? Sometimes they are not published." - Joe M.
“I have found the easiest and most user-friendly method to find out about preferred routes is to use one of the online flight planning tools such as Forefight or Fltplan.com. After you submit your routing and altitude request, they return a list of routes that ATC has actually assigned to aircraft between the two airports requested.
Fltplan.com will also list routes by type of aircraft.
(If you use Fltplan.com), look at the routes and aircraft types carefully. It’s not uncommon to find different routes depending upon type of aircraft – for example, pistons, turboprops or jets.
Another source may be checking with a local pilot or flight school. Ask them what is the usual route out of here in the direction of your destination.
The FAA publishes a generic index of preferred routes between certain city pairs in the Airport Facility Directory (The Green Book). I found these of limited value and can be cumbersome to use.
The FAA also has an online database for accessing IFR preferred routes. The database interface is not the easiest to use. If you get the input syntax incorrect by one character, it will return – No route found.”