SO hinted that he's ready for a baby.

Yesterday my SO and I were talking about our future as we always do. Normally we talk about how we aren't ready for a baby yet and how we have time to wait (we kind of have time, but my health problems can affect fertility). Well yesterday we went about the conversation normally until we got to the baby part where he said he'd be okay with a baby right now. It shocked me a bit, because I always thought I'd be ready before him. I'm still not ready and he's fine with that, but now that I'm thinking about it more, how do you if you're ready?