Check torrent port; how to check if your torrent port is open; check if your torrent port is forwarded properly

Some time ago there was an online tool from utorrent to check whether your port is forwarded properly. The service has been moved, however. Nowadays this functionality is built into utorrent.
So what should you do if you use a different torrent client? - e.g. Transmission or a torrent client on your router.

There is still an online solution for this but it's hard to find the proper site. One of the services that actually works is:

http://www.canyouseeme.org/

Have fun with it.