GW2, how to distinguish between the players and NPCS!

When players do not move, you can hardly distinguish him is the player or NPC! So far I’ve used two methods:

1. Look for a guild under their name
2. Look for a class icon above their portrait                                                                               Method one doesn’t always work, however, because not everyone is in a guild. I thought that method two was foolproof until I came across this character with a warrior icon who was walking along a patrol path much like an NPC guard would.                                        Is there a way to tell with certainty whether a character is a player or an NPC?