How can I check if an Open Access Journal is a trusted one?
If you choose a journal listed in DOAJ it is a trusted one. Furthermore you can visit think.check.submit and learn how to identify trusted journals and publisher.
Where do I get an overview of Open Acces Publications at MPI?
Which license should I choose for my Open Access Paper?
The Max Planck Society has signed the Berlin Declaration on Open Access. Following this statement the CC-BY License would be the best (& most open) one. But it is your choice as an author which one you want to use. Please use the CC License Chooser to find the appropriate license for your work.
Is there an Open Access Guideline at MPI Magdeburg or MPS?