How Do You Recognize When You Need a Boundary?

I think saying yes to boundaries, which is a weird way of phrasing it, but I think prioritizing the things that I need to make myself happy. How do you recognize when you need a boundary? Usually for me, I recognize it when someone's overstepped it and I'm like, oh no, I'm upset. You're upset. Are you resentful? That is my always my trigger. So what you'll say yes to then essentially is putting up more boundaries or to get where you want to go.