Examine a diaper in the current diaper size to see how it fits your baby. If you notice red marks around your baby's upper legs and tummy from the elastic in the diaper, the diaper is likely too small.

Additionally, if you notice that the diaper looks or feels too snug on your baby, it is likely too small. You may also begin to notice leaky messes if your baby's current diaper size is too small.
Try the next size up in diapers to see if it absorbs better and fits your baby more comfortably. If leaks stop and it looks like a better fit, your baby was most likely ready to move up. If you notice excessive gapping at the legs and waist, the diaper may be too large for your baby.