Often times you may need to replace your rotors for several reasons warping, glazing or grooving. Here's some ways to spot problems, so you know when to change them.
Warping is the easiest problem to spot. This will cause a slight to sever shutter when stopping.
Grooving is the second easiest to spot, this is caused my faulty wearing of defects in the pads. To check simply run a finger across the rotor (while cold) and check for any deep pitting or grooves. Grooving will also represent itself sometimes as brake squeal/whistling also, but can be mistaken for pads that need replacing as this will make a similar sound.
Glazing is the hardest to spot. For this you'll need to take the wheel off and check for a highly polished look or discoloration. This is caused by excessive hard stops and is usually accompanied by a slight or heavy warping as well. This will cause brake fade and longer stopping distances, since it will cause the pads to not grab as well.
Side Note - While they used to "turn" rotors (shaving them down), this isn't done anymore do to tight tolerances and materials of todays vehicles. I highly recommend purchasing new rotors instead of having them "turned"