Methods and apparatus to test a subscriber line for a broadband access service are disclosed. An example network interface device (NID) comprises a tone generator to transmit a tone on a subscriber line to characterize the subscriber line while a DSL modem is providing a DSL service via the subscriber line, the tone having a frequency occurring within a range of frequencies in use by the DSL modem to provide the DSL service, and a pair of terminals to couple the DSL modem and the tone generator to the subscriber line.