One of the things I often struggle with is the difference between what my wife and I find appropriate.
For example, I am a big fan of public displays of affection. I love my wife and I do not care who knows it or where I show my affection.
She does not share my affinity for contact in public.
There are other, more complex differences. Sometimes those differences are quite valid and her position is the logical one. Other times the events of her past weigh on her.
So the challenge I face as the Domiant is to create that safe space where I can challenge her without pushing the limit too far. To create a poor comparison, it is like working with children. You want to set them up to be strong and confident people without pushing so hard that they close down on you and hate you forever.
My best tool is knowing her. We have been together almost 20 years, so I know her, I know what she can do and what she cannot. She is much stronger than she believes.
I have to push.
I have to watch her eyes, her body, the way she reacts to touch, activities, situations. I have to listen and hold back when we get close.
Safe words help as well. Sometimes she can fool me, so having a warning and a safe word keeps her safe. It can also give her the confidence to go on.
The best tool to identify a limit is practice. Start small and work up from there. See how much you can push, how far s/he can go; and be ready to stop and provide care if the line gets crossed.
For Dominants who are trying to catch up with their submissive, the oppsite can be true. You might be tentative to go to their limits. Ask questions, have the submissive help you grow, and continue the adventure!