A system and method for testing lines to determine service-affecting conditions on a line, such as telephone wiring. Conditions may be identified from low frequency variations in a frequency spectrum measured on the line. The line may be stimulated with a broad spectrum signal, such as may be generated by a DSL modem operating in the L0 state or may be the result of noise on the line. Analysis of the low frequency variations may yield information about the nature of service-affecting conditions allowing conditions, such as un-terminated or poorly-terminated extensions to be identified. The method may be performed by a test device attached to premises wiring in a telephone network and used to determine whether the premises wiring will support VDSL2 or other high speed data services without modification or may indicate changes at the premises to enable the wiring to support high speed data services.