Potassium Sorbate -- Bin it. It has a relatively short shelf life (~2 years), and if it's degraded you'll spend far more uncorking the bottles to let them finish fermentation, then buying corks to re-cork.
If it were me, and I thought the remaining items are more than 2 years old, I'd bin 'em. However, if you're adventurous:
Consider appearance. Powders should be powdered, not clumping, and should not be discolored. If anything looks odd, dump it.
Campden tablets -- If they are kept dry in a sealed container and have no sign of crumbling, they're probably ok. As a test, crush one and dissolve in 1 cup water. Sniff VERY GENTLY -- if it reeks of sulfur, it's good.
Wine Tannin -- some sources say to use within 6 to 12 months once the package is opened.
Pectic Enzyme, Acid Blend, Yeast Nutrient -- Some sources state if it's been kept dry and sealed, and looks fine, it's probably ok.
I make it a habit of writing the date on the package when I buy any additive, and do have the bad habit of keeping some things longer than I probably should.