There are two primary ways steel gets into the feed supply. A silage chopper can pick it up and cut it into fine bits or a hay baler picks up a stray end of wire and running large bales into a tub grinder to more easily mix feed stocks into a total mixed ration will chop the stray wire (and other parts) into moderately short bits.
Many feed wagons have strong magnets under the discharge chute and grinder/mixers (used to grind grain and supplements) have likewise strong magnets just before the material enters the hammer mill. Despite that some bits will slip by due to the volume of material passing over the magnets as it comes out of the feed wagon.
We do have the vets administer magnets to the heifers we keep at around 9 months of age. One has to watch to be sure they don't work them back up and spit them out! A couple of years ago we had some that did that.
Symptoms of hardware is usually a cow "off feed" meaning she is not coming to the bunk to eat with the rest but usually stands apart and in acute cases will become hunched back. A veterinarian can usually diagnose it with a stethoscope as I think the breathing becomes labored in more acute cases.