HD, let me start by saying that my cattle have Wind and Rain loose minerals under cover in their loafing shed along with very high quality pasture for most of the year, then some high quality hay in the dead of winter. Grass and hay does not store Selenium.
Up until a few years ago, I gave Muse injections 2x per year. But Muse only lasts in their system for 30-60 days. Thinking that I was ok, I stopped giving it to the older cows and cut back to just giving it to the newborns.
My cattle never had the typical signs of Se deficiency in older cattle. No retained placentas, no fertility issues, ect.
What they did develop, (one at a time) was lameness, in coordination, and weakness in the back legs. I would pen up the affected cow, examine feet, treat with Aspirin or Dex for an injury.
I lost a cow two years ago that progressed to recumbency, and died. She had blood work done, but the vet did not check a Se level.
This last young cow, (in addition to the above lameness and weakness) would "duck walk" and collapse in the rear legs when she tried to hurry somewhere. She is the one that I did a battery of tests on, because I wanted (and had) to know what was going on! This was over a period of a year.
Mean while, another older cow came up lame in a rear leg. Again, suspected an injury. But it seemed to switch legs........Hmmmmm something weird is going on!
The rest of the story is told in the beginning of the thread.
Yes, your vet can send blood samples to check the status of their mineral levels.
I agree with loose minerals "being scary". Obviously without periodic testing of a sample group of your cattle, you have no idea if they are consuming the proper amount.