If the slave does not respond, the master will read a NACK on the address or the data frame (ANACK or DNACK). Then you will get an event with type NRF_DRV_TWI_EVT_ADDRESS_NACK or NRF_DRV_TWI_EVT_DATA_NACK if you are using event handler. If you are not using event handler (using blocking calls), then the rx or tx function will return NRF_ERROR_DRV_TWI_ERR_ANACK or NRF_ERROR_DRV_TWI_ERR_DNACK.
