How do I know if the Organization I'm now the admin for was built in Classic or Lightning?

@Justin Bolner​  Yes, you should be using Lightning. Check out this trailhead for guidance. https://trailhead.salesforce.com/content/learn/trails/lex_admin_migration

And this help documentation.https://help.salesforce.com/articleView?id=lex_welcome_to_lex.htm&type=5