Here is the stats on Graveknight Armor;
http://www.d20pfsrd.com/bestiary/monster-listings/templates/graveknight-cr- 2
QUESTIONS:
What number does the skill check need to be, and how do you figure out the skill check #;
Basic info
Basic info +1 additional item
Basic info +2 additional items
Is the skill check a arcana or religion or both?
"Bueller, Bueller? Anyone there?"
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Both answers can be found under the Knowledge Skill in the Core Rulebook, page 100.
It is Know Religion, because it deals with Undead. (both the armor and the creature).
Because the Grave Knight is a rare monster, the DC is 15+ it's CR.
The example 10th level Fighter Graveknight has a CR of 11, so the DC is 15 + 11 (26), (31 & 36 for more information).
Know Nobility & History should also be able to help identify the lore of the individual Graveknight's armor, (identifying it's original style or symbols for example), but Know Arcana should be useless. Arcana is not a catch all skill, it deals specifically with "ancient <arcane> mysteries, magic traditions, arcane symbols, constructs, dragons, and magical beasts". Know Religion on the other hand deals with "gods and goddesses, mythic history, ecclesiastic tradition, holy symbols, and undead".