Epsom Salts contains magnesium sulfate (10%) and Sulfur (13%) and can safely be used as bath salts for treatment of skin disorders. On the other hand, Aluminum sulfate, when mixed with water, forms Sulfuric Acid, a highly toxic solution, whose vapors can irritate the eyes, nose and respiratory system.

A tiny amount of your unknown powder, mixed with a very small portion of water, in a glass vial, or other suitable container, should provide an answer. The odor of Sulfuric Acid will be unmistakable!
Use extreme caution when performing the test! If it is Aluminum sulfate and produces Sulfuric Acid, do not allow it to come into contact with your skin or clothing.
Rb

Any Epsom Salt product I've purchased hasn't been actual powder, but more like tiny crystals somewhat larger than table salt but smaller than coarse sea salt.

And if I had an unknown product, gift or not - I'd not think twice, I'd throw it out.

I just want to point out that Sulfuric acid is a hazarous waste and you cannot simply pour it down the sink or through it in your garbage can. If your friend gave you this powder they could have been saving themselves a trip to the hazardous waste disposal site. here is an exerpt I found on line: Disposal
Material is regulated as a RCRA hazardous waste. Contact the Waste Management
Group for specific disposal requirements and procedures. Containers and other materials
that are contaminated with sulfuric acid must also be treated as hazardous waste.