DO NOT relie on the above!
as an ISP i use the only true intrusive method.
Test Result
this runs an intrusive through check onthe line integrity also updating your sync level, it can only be used once a day as it is NOT meant for domestic users. basically it calculates at LINE level whats going on, SNR and ATTENUATION on the line, and mst importantly line length. ADSLguide and the like relie souly on server up and down rate speeds, the server you up and download too can only give results in conjunction with its own load, IE if the server has mod to hi load you WILL NOT get an accurate result!
the above tool checks sync details with the DSLAM in your exchange and server overheads ARE NOT a contributing factor.
Hope this helps
Robbie