This can be seen on your own router - usually under DSL status or statistics.
#edit - Found the following from another post that sums it up pretty nicely:
Noise Margin (AKA Signal to Noise Margin or Signal to Noise Ratio)
Relative strength of the DSL signal to Noise ratio. The higher the number the better for this measurement. In some instances interleaving can help raise the noise margin to an acceptable level.
6dB or below is bad and will experience no synch or intermittent synch problems
7dB-10dB is fair but does not leave much room for variances in conditions
11dB-20dB is good with little or no synch problems* (but see note below)
20dB-28dB is excellent
29dB or above is outstanding
* Note that there may be short term bursts of noise that may drop the margin, but due to the sampling time of the management utility in your modem, will not show up in the figures.
Measure of how much the signal has degraded between the DSLAM and the modem. This is largely a function of the distance from the exchange. The lower the dB the better for this measurement.
20dB and below is outstanding
20dB-30dB is excellent
30dB-40dB is very good
40dB-50dB is good
50dB-60dB is poor and may experience connectivity issues
60dB or above is bad and will experience connectivity issues