A lot of this comes down the the nuance of the dynamic you're have with your dominant partner. The only "correct" way to learn your limits is through clear communication and mutual consent. To help identify nerves, I would suggest agreeing to a secondary safe-word for when you're unsure how you Read more… feel about something as a way to communicate to your dom that they should ease up, but you don't want them to fully stop.
As for knowing your fit with a Dom, that's not easily told until you hit that moment where they earn your trust. Any Dom worth their salt will respect your limits despite their own wants or needs. The Dom and Sub titles are contradictory to their nature; the Submissive is only Dominated because they allow it, and the Dominant is a slave to the Submissive's limitations/concent.

I believe it depends on the individual dynamic of each relationship. Technically, I guess it boils down to how well each party copes when needs go unmet, whether because life gets in the way or because moods don't align.
You find out a lot more about a partner when you live with them. Sometimes Read more… familiarity can breed contempt; you may learn things about them that put them in a less attractive light. Other people might have the exact opposite effect and become closer than ever. It's really up to you and your partner to decide how you're feeling about your relationship/dynamics and what would the 'next step' be, or if you even need one. It's perfectly fine to enjoy what you have as long as there isn't any unspoken expectations that may or may not be fulfilled