How do I tell if an NPC is essential?

According to this wiki of Skyrim console commands, you can select the NPC in question (generally using your cursor) and enter the command IsEssential which will return 1.00 if they are, in fact, essential.

Related functions: To check whether an NPC is essential to begin with, select him/her in the console (so that the ref ID appears), and enter IsEssential - if the console returns "1.00", the NPC is essential.

If you have 100 points in Pickpocketing and the perk that allows you to pickpocket equipped items, you can test them. You will not be able to remove equipped items on essential NPCs, even with the trait.

Source: http://www.uesp.net/wiki/Skyrim:Essential_NPCs