Hey spiderbob,
To check if an individual card in the library is equipped or not, you will want to use:
[Game_Card].origin()
This is will access the origin value of a Game_Card object which will return "learned" if obtained will Initialization or Skill Learn, "equip" if it was added through Equipment and "state" if it was added through a State on the Actor. You can find a Game_Card by accessing any collection of Cards such as:
$gameParty.members()[0]._cardDeck._data[0]
This gives you the top Card of the Deck as a Game_Card object which you can access the origin of using the above function.
To check how many of a card you have in the library, the script call you are looking for is:
[Game_Cards].amountOf(skillId)
This will return the amount of cards matching the Skill ID you pass in of the Game_Cards object you are calling it from (which is just any collection of Cards). A couple that might be useful to you:
[Game_Actor]._skillCards
This will give you all the cards an individual Actor has including ones they have in their Library which are not in their Deck.
$gameParty._allCards
This will give you all the cards in a Party's shared library if you have that plugin parameter enabled.
Hope that helped. Let us know if you have any further questions. We would be happy to assist :)
Cheers,
MythAtelier Team