How do I ask a professor whether his seminars & lectures will be available to students from other departments? I will apply for a PhD soon. I would be applying to the Politics department, but there is a professor in a different department who holds really interesting seminars on political topics. However, as I said, he is not affiliated at all with this department. How do I ask him / how could I phrase this best?

PS: My question is not whether I can attend his lectures as a guest -I would love to take these seminars for credit, if I can. I just don't know whether that's an option, or whether students from the politics department will be allowed in these lectures/seminars.
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.