You can see a partner's true character by how they treat their friends or elders.

If you hang out with your partner's friends and get somewhat close to them,
you can get a general idea of what kind of crowd they associate with.
You can also tell by observing their behavior and speech when they interact with elders.
Unless a relationship has lasted a very long time, most people hide their true selves while dating,
so I see many people regretting their marriage decisions because they didn't consider this at the time.