How can I tell if these symptoms are due to my recent surgery or dementia?

Alzheimer's disease is also a brain disease, so it is not surprising to me with your history of brain tumor and surgery that you would have similar symptoms, such as memory loss. You do not mention in what part of your brain your tumor was removed from in your question. Different parts of the brain have different functions, so your neurologist should have told you after your surgery what symptoms to expect. If memory loss wasn't one of them, I would make an appointment with your neurologist and let them know what symptoms you are having. I am not really concerned with Alzheimer's being your problem due to your young age, but you may have some new brain problem or complication from your surgery which may need to be evaluated. Good luck!