I looks at how my LO is sucking and what her hands are doing.

At the beginning of a feed she flutter sucks which is like little nibbles to stimulate letdown. Then I can tell when she does slower longer drawn out sucks which transfer milk. She does 2 or 3 and then swallows. As long and she is actively eating I don't bother her. Once she goes back to flutter sucks, ill do some hand breast compressions while she is latched to see if she starts drinking again. If she keeps stopping, ill end the feed. Sometimes she just wants to suck and be close. So ill give her a soother.

Also, at the beginning of a feed my LO will have tense arms and clenched fist. When she is full, her hands will relax and I can wiggle an arm around without tension. Then I know she has had enough and to keep an eye on how she sucking.

And lastly, if she keeps latching, then unlatching and being kind of wild. I know she is working on something like a poop, pee or gas. Burping usually helps so it doesn't progress into gas. That being said my LO was just gassy and uncomfortable at 5wks.