A good way to tell if a Luvs diaper or any other disposable diaper is wet is by grabbing the front of the diaper with your hand and squeezing it. If it feels squishy then it's usually time to change it.
A wet disposable diaper will swell up and get squishy because of the SAP super absorbent polymer that is used in the disposable diapers to absorb the urine.