Is it possible in some way to check if a IP is a JonDonym node? (Regardless of exit policy, the check should return true for even non-exit JonDonym nodes)

You may compare the IP's in question with the ones in your jap.conf. There are all the "values" of the mixes listed (of course including their IP's).

Where can a updated list be fetched? Or how often does the mix list change?

The mix list does not change very often. And if so, your client (i.e. JonDo) incorporates the new mixes or cascades automatically and saves them to your config-file while you are shutting JonDo down. Thus, you'll find the new ones almost immediately in your jap.conf as well.

How often is the list updated? Monthly? Yearly?

The list updated automatically, but it change not very often. Less than one per month.

There is a list of exit mixes online: http://infoservice.jondonym.com/exitaddresses

[quote=cane]There is a list of exit mixes online: http://infoservice.jondonym.com/exitaddresses[/quote]

Diese Seite wirkt auf meinem Bildschirm vor lauter Code-Zeichen recht unübersichtlich. Sie kommt mit der Fehlermeldung:

"Mit dieser XML-Datei sind anscheinend keine Style-Informationen verknüpft. Nachfolgend wird die Baum-Ansicht des Dokuments angezeigt."

[quote=cane]There is a list of exit mixes online: http://infoservice.jondonym.com/exitaddresses[/quote]

This list is presented in a rather strange way. It comes with the following error:

"Mit dieser XML-Datei sind anscheinend keine Style-Informationen verknüpft. Nachfolgend wird die Baum-Ansicht des Dokuments angezeigt."

[quote]This list is presented in a rather strange way. It comes with the following error[/quote]

There is probably something wrong with the CSS style sheet. Maybe the JonDos team will look into it.

[quote]Maybe the JonDos team will look into it.[/quote]I don't think so. All the needed information, i.e. the IP addresses (and the ID's) of the exit mixes, is clearly visible.