Is that the Max distance you have used so far for a digital line, or is it the maximum distance you could get one to work?? If you just haven't tried further than that then you have a chance, but it that is the furthest you could make it work, expecting that distance to double is unlikely.
You can purchase test equipment that you can connect to the line to determine how long the cable run is, as long as there isn't anything else in the line like a load coil or something that might affect the test.
My suggestion would to just hook the sucker up and see if it works - if it works is all you really care about. Using heavier cable will usually help, and twisting two pairs together (or laying your cross connect wire across 2 pairs) usually will help too. Be sure to also use heavier cross connect wire and not the cheap skimpy stuff.