Why? What does extra selenium do that is bad?
Excess selenium has been shown to increase viral virulence. It makes them stronger. Selenium levels obtained from plants and animals are highly variable. One farm directly adjacent to another farm could have a 1000 fold difference in selenium levels from their plants or animals. Too little selenium is bad but too much is bad also. Studies showed that eating a handful of brazil nuts per week was detrimental to long term survival but eating only a couple brazil nuts a week was beneficial to long term survival.
On a side note, the selenium obtained from plant sources is in the form of selenomethionine. Selenium obtained from animal sources is selenocysteine. It seems the animal source of selenocysteine provides higher and perhaps more consistent levels of selenium(because of lower pH levels) but even that is very inconsistent. One steak could vary greatly in selenium content compared to another steak.
How do you know if you should supplement or not?
Blood tests would give the amount in your blood, but selenium is primarily supposed to be intracellular. It has suppressing effects on cancer cell growth so increasing its intracellular content would be beneficial. In someone with inflammation, extracellular polyamines are increased as is their turnover. This increases extracellular selenium as well as selenium loss. Since selenium (actually selenoproteins) are used in making type l iodothyronine deiodinase and in glutathione peroxidase, it is extremely important to have adequate levels. IMO, I think we have issues with thioredoxin because of our increased oxidative stress. This would alter the reduction of thiols in cysteine which would mean eating animal protein is more detrimental to us than "normal" people. Thioredoxin reductase is responsible for selenium metabolism and it may be altered for us as well. I would get tested for selenium but in general, we may need more. If you have viral issues, you may need less. I do know one of the reasons we get sick is because of the decrease in elements (nutrients) we need for our pathways to function. This is often caused by our pathogens but doesn't have to be.