How to know breast is empty?

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Hi there, have a 13 day old little boy and am exclusively breast feeding. I'm really paranoid that he's sleeping too much and not feeding enough. How can I tell when to change to the other breast? Im afraid that I'll change too soon before he gets the hind milk or that the breast he's on is emotying out.
Thanks

Thanks for all the reassurance. After a hour and a half feed this morning, he was awake and alert for almost 2 hours. His (doting) Grandad thinks his fingers are getting pudgier so that's good enough for me!
Thanks again