You can keep a track of the dubbed episode DVD releases on Wikipedia where you can see that the dubbed versions of Episode 422-433 will be released on December 15, 2015 and the dubbed versions of episode 434-445 will be released on January 26, 2016.
The dubbed version is released online 2 weeks before the DVD Sales go out. You can find them uploaded on Funimation site or other sites.
You may also keep track of the DVD release dates on One Piece Wiki where too, you shall find the same information.
A final alternative is this site which is not only for tracking the release of the dubbed episodes of One Piece but also other anime.