alli - I think with a bit of time one gets better at reading your babies signals, and differentiating between their cries. It helped me to keep an eye on the time - not from a rigid control perspective but for example, with eating - a baby at that age generally needs to feed every 3 hours, so if its been 2.45hr since his last feed and he's fussing - there's a good chance she's hungry.
Same as with sleeping - again he probably can't manage being awake for more than 1h30-2h00, so if he's been awake for that long, its probably a good idea to put her down for a nap.
My dd always starts to rub her eyes and pull at her ear for example when she's tired, you will get to know the signals of your ds.
The most helpful (and relaxed!) book I read about this is called Baby Sense, it explains a lot about what babies need and how to read their signals, and gives guidance as to an appropriate flexible routine for each age.
A link to it is here
www.amazon.co.uk/s/ref=nb_ss_0_10?url=search-alias%3Dstripbooks&field-keywords=baby+sense+book&spref ix=baby+sense&x=17&y=20
Also you can check out their website for some high level info www.babysense.co.za