[header] How to tell how old a kitten is [title] Look for an umbilical cord stump. [step] If you find one, you can be sure you're dealing with a newborn kitten. [substeps] The mother usually bites the cord until it breaks. 
The remaining " stump " will be a small piece of tissue hanging off the kitten's belly. Cats' umbilical cord stumps usually fall off in the first 3 days of life.