I don’t know why you switched the order here around the の here. It would still be 社長の車, even if you move it to the topic position.
You can ask either way, but the way you’ve been discussing things to that point would determine which one is natural, because は is either going to represent a topic that’s already been discussed or a contrast with something you are making a point not to ask about.
BTW, referring to a 社長 as a “manager” is underselling their position a bit.
You could get away with saying something like this if you pause between 車 and 社長, but for grammar learning purposes it’s a good idea to have a particle in there.
So, in summary, if you know how to use は and が based on topics of discussion and contrast, then it should be possible to know which one to use.
I look forward to the day when I can write a simple sentence in Japanese without making silly typoes or easily avoidable mistakes. But considering I still fail with English occassionally, it will probably be a while before I get there.
Thank you very much for the help, Leebo. I think I understand better now.