Well, my mom had a coworker with a brain tumor and he was having seizures which was the tip-off. So, you can't miss that!
Also, think headaches so bad that you physically can't lift your head off the pillow. And even then, it is not necessarily a brain tumor. My grandmother had such bad headaches that she was hallucinating and they did a CT scan and MRI which turned up NOTHING. It ended up being migraines. That was 25 years ago and she is still fine, just an occasional migraine.
Long story short, they are really rare. As for frequent urination, do you take birth control? The BCP made me have that. Also if you are drinking a lot of caffeine or alcohol, or even just stress.