Your ISP's tech service should be able to check your DSLAM stats to see what sort of line noise levels you have.
Also, you can usually telnet to external ethernet modems/routers/bridges to see what your line noise is (far and near), whenever you want, and in the comfort of your own home.
But, you'll probably need your ISP's help to do this, and many don't support this action. What kind of modem do you have?
The problem is interpreting the numbers. Generally you'll get a signal-to-noise ratio called a "noise margin", measured in db (decibels). A noise margin of 6-7 db is bad, and you generally can't keep a sync below that that. A good noise margin is greater than 15 db.
The db measure is on a logarithmic scale. So, a 6 bd noise margin is 10*log(4)=6, meaning there is only 4 times more signal power than noise. A 15 bd noise margin is 10*log(32)=15, meaning there is 32 times more signal power than noise.--