Honestly I do what everyone else is saying, I open my eyes.
However, finding a good place to really "Train", not just play around at for a bit till you get in someone’s way or you're asked to leave, is a different story. If you are looking for spots to really drill moves I suggest: Local schools. Elementary schools are good because of the playgrounds and lack of security. High schools are good because they usually have more walls, rails, planters and sometimes odd architecture. Colleges are by far the best though, they always have fun designs and progressions, only problem with high schools and colleges are security.
So your best bets are gonna be local schools with no fences or security or Playgrounds.
Besides that, Parkour is like women; if you learn some moves and get confident, the world will almost magically open up to you.
Good luck