I think it’s more than just the mirror given to Regina by the Genie. In magic, mirrors and other reflective surfaces are supposedly a means of divination, precognition, and in some cases transportation. So, however one acquires magic in the Enchanted Forest, a magic user would be able to use a mirror to see other places. Though not yet shown, Rumpel may be able to do this as well, however, at the same time he is not able to shield himself from Regina viewing him, as seen in ‘Skin Deep’ where Rumpel has his mirrors covered.
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