Epsom salt is not a complete fertilizer (one that contains nitrogen, phosphorus, and potassium), so you'd want to use organic (compost, fish emulsion, blood meal) or inorganic fertilizers (granular or water soluble under a variety of brand names) that have a nutrient analysis that has three numbers such as 10-10-10 which indicates the percentage of N-P-K (nitrogen-phosphate-potash) in the fertilizer.

A soil test is always the best place to start. While SDSU no longer has a public-facing soil testing facility, North Dakota State University and the University of Minnesota each analyze garden soil samples (for a fee) and provide recommendations on the necessary amendments and appropriate amounts. Information on how to submit samples to them can be found at these websites:

https://www.ndsu.edu/snrs/services/soil_testing_lab/

http://soiltest.cfans.umn.edu/

Epsom salt may be used as a fertilizer IF your soil is low in magnesium. Crops such as pepper can be more sensitive to magnesium deficiency in soils - this would appear as interveinal chlorosis on older leaves first (with symptoms on new growth as well if the deficiency is very severe).