How can you tell whether or not a gem or piece of jewelry has magical properties? There is an asterisk next to two gems and one piece of jewelry I have, but I'm not sure this necessarily means they are enchanted.

If examining gems, they will have any enchantments listed. For example, there are some gems that may come with spells and you will be shown if they have one or not(usually not). In terms of defensive items, +x rings, crowns, etc they will list whether they have any bonuses or not.