There is no feature on the forums that tells you which world a player is on. Unless a player indicates in a thread or their signature, the only other way is for you to private message them here on the forums.
You can click on a players name, which brings up a small box. Click on the "start a conversation" link, this allows you to private message someone.
or ...
Hover over your name at the top right to open a pull down menu, click conversation, enter player's name, enter your message and then send it.