How do you know when your done having kids?

Okay for starters I'm like 75% sure I'm done having kids (which breaks my heart) lol but anyways how do you make the final call? What if in 3-4 years I decide last minute I want another one? I'm only asking this because I want myself or my husband to get fixed when we're for surely done because my body doesn't agree with birth control. I have 2 beautiful kids, the perfect family a boy and a girl. I love kids though and always pictured having 3. But on the other hand Idk if I wanna bring another one into the scary world we live in. I don't think my husband really minds either way, but he is 3 years older than me so if we did decide to have another one later on he'd be pushing 30 years old (which isn't old I know) but he has plans of retiring at 45. Idk for the most part I feel like we're good but then there's that "what if" what if we are meant to have another one. I won't make the final decision till I know for SURE, but how do you know?