For my daughter, she sucks on her thumb really hard when she's hungry, so that's how I know. Babies put toys in their mouth because they are curious and using their mouth to explore is one of the ways they learn about the object. They will also cry to alert you if they are hungry. If you really can't tell, feed your baby every 3 hours.
Do you mean you need to use a toy to distract baby to eat? Normally baby will turn away his/her face when she feels full (either to left or right). If he/she is still hungry, he/she will continue to open his/her mouth for food. Sometimes if you find baby's tummy is already very bloated, you should stop offering food.
thanks