I recently purchased a 2010 Escalade which came with an extra battery in the engine area. I've never had a vehicle with a spare battery. It is still wrapped in plastic. How do I know if it is useable? And wouldn't it have to be charged occasionally so that it would be a useable spare in an emergency situation??
Only way to know is to check it and test it.
Is it filled with fluid - electrolyte to the correct level - just above the plates. If it does not have any fluid in it, then it may be a dry-charged battery that needs filling to be ready for use, which probably means it is in “as new” condition. Especially as you mention it is wrapped in plastic.
What voltage does it show at the moment - if it is too low then it may not recharge or recover and that means it is scrap.