I am renting a car in Berlin and driving to Szczecin and further. What insurance questions, if any, can I expect at the Germany/Poland border? I do not want to purchase CDW coverage from the rental company. Will Polish border accept my Mastercard credit card insurance coverage or will Polish border require something more explicit? Do they check at all?

Someone with more experience may well chime in here but a couple of observations: you won't get stopped at the border because both Germany and Poland are signatories to the Schengen agreement. And your rental company may well frown on you taking your vehicle to Poland. Some years ago when we rented a car one of the terms of the agreement was that we should not take the car to Poland. At that time the likelihood of theft was quite high. I don't know what the current state of play is nowadays.

Lavandula

No problem with the rental car company.

Your rental car will have mandatory 3rd party liability insurance. So if you cause an accident, the other party gets reimbursed for damages.
If and how you insure your own liability to bring back the rental car undamaged and in one piece is your choice.

Just make sure (e.g. by browsing the other threads on declining the rental car company's CDW) that you know how your 3rd party insurer (MC) will handle pending or disputed claims.

After an accident, the rental car company will not wait until you have settled the claims with Mastercard insurance. They will get the car repaired and invoice you.
Unless MC is obliged by contract to jump in and pay the rental car company on the spot, there can be a time lag between the invoice of the rental car company for the damage repair and the reimbursement by MC insurance.

Cowboy, thank you for the reply. I have rented cars in Europe on a number of occasions and am familiar with the CDW question. What I am interested in is the border question and whether my Mastercard coverage will satisfy the Polish border.

maybe that part got blurred in my reply:
The Polish border control (IF there is a random spot control you cross it) only cares about your passport, and IF border control asks for insurance they only care about 3rd party liability insurance.
You will have proof of that in your rental car papers (it's a green leaflet, you might have noticed it on prior rentals).
Your MC coverage or the whole CDW issue is of no interest whatsoever (except for you, of course).

Thank you Cowboy