Epsom salts here in US...will be labeled for soaking and oral use (if they are clean enough to use that way). The bag I have says
"magnesium sulfate USP" which in the US means pharmaceutical grade.

So check the package for labeling. I realize other countries may be vastly different, but most I would think would label the contents clearly.

I seem to recall some posters from UK at one time saying they had difficulty finding it easily. Here in the US they are in most pharmacies and grocery stores in the health section.
With different cultures the "name" of the product might be different, etc.

Using magnesium sulfate internally will have significant laxative effects, so be careful with that aspect. A laxative response means the dose is too high for supplement use.

This is an example of a product that is pharmaceutical grade:
http://www.amazon.com/Epsoak-Epsom-S.../dp/B004N7DQHA

This link is rather expensive... Most USP epsom salts here in US are very inexpensive IMO. I get mine at Costco (a discount members type warehouse).