Welcome to the bipolar board and HealingWell.
I will try and give you my take on your questions, but it's really important to know that the only person who can determine if someone is bipolar or not is a doctor. If you have a loved one who is exhibiting signs of bipolar, try and convince him or her to get to the doctor. There is no "test." There is only one on one conversation with a qualified doctor. A psychiatrist is best.
A person can have signs but not be bipolar because the elements of bipolar can be elements of so many other disorders as well. General anxiety disorder, depression, ADHD, schizophrenia, panic disorder, borderline personality disorder: all these mental disorders share symptoms with bipolar. That's why it's important to have a doctor diagnose you.
Relationships absolutely can work, but they take more effort than your average relationships. I'm happily married but I work hard to keep myself stable and my relationship healthy. My husband is a VERY patient man: he doesn't take things personally when I'm snappy, he often has to pick up extra work load, I can't work a full time job, and my mood can be unpredictable. Sometimes he doesn't know what "mood" he's coming home to. That requires work on his part to put up with. But he obviously thinks I'm worth it. (Thanks, DH!)
I hope this is helpful. Keep writing, keep asking questions. We're a friendly board and like to help.
Co-Moderator, Bipolar Forum