How do I ask a professor whether his seminars & lectures will be available to students from other departments?
The answer to your question is easy. Simply ask: 


  I am very interested by your lectures. Can I attend them as a student in the politics department?


The professor, however, is probably not the best resource to answer your question, especially if you want to enroll in the course. You should enquire at your future department what is allowed and what is not. Rules can vary between departments and programs.