A babies diaper will be too small when it takes a little extra effort to stretch the tabs to secure the diaper.
Also if you notice any red marks around the babies thighs or waist then that's either an indicator of a diaper being put on too tight or the diaper is too small and you need to go up a size.
You should be able to place two fingers easily inside the waistband of the diaper when the diaper fits just right and is not too tight.
Another way to tell if you need to go up a size in diapers is if the baby is leaking through the diapers as a bigger diaper will hold more pee.
The weight rating on the diaper packages is just a general guideline so sometimes you might need to change to a larger diaper size even when the baby is within the weight range of the diapers you're buying.