In some cases this can be quite difficult, for the unskilled eye, but normaly they are thicker than a standard BT cable, you used to be able to look at the poles and Identify a BT cable from a mains cable, all mains Telegraph poles were marked with a Warning overhead power line sign, but now the EU has made it law that all BT telegraph poles should also show the same sign even if it does not carry a mains wire.
Just take care and check the area your working in, if in doubt stay away or take special care.