Do Guards/NPCs Respawn

Generic NPCs will respawn some time after they die. These include generic guard NPCs and the following NPCs:

• Farmers
• Fishermen
• Hunters
• Imperial Legion soldiers
• Nobles
• Peddlers
• Stormcloak soldiers
• Thalmor
• Vigilants of Stendarr
• Afflicted
• Bandits
• Dremora
• Falmer Servants
• Forsworn
• Thugs
• Vampires
• Warlocks

List of all re-spawning NPCs

Most named NPCs, on the other hand, are either 'Essential' (unkillable) or will not respawn after they have died. (Some named NPCs are killable and respawn - see the list of all re-spawning NPCs.)

Source: UESP article on Skyrim NPCs

Yes they do, if you look them up on UESP you can see if they respawn or not.

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