Throughout the series we have seen many scenarios in which someone acts as someone else. Barty Crouch Jr. transforms into Mad-Eye Moody, Harry and Ron transform into Crabbe and Goyle, and Hermione transforms into Bellatrix. This is a huge flaw in the series, as you potentially have no idea who you are talking to, or oblivious to the fact that someone could be using Polyjuice Potion, to transform into another.
My question is, how do you know who you are really talking to? It seems silly (before Voldemort took over the Ministry) that you must ask everyone a secret question to identify them. We see Remus do this to Harry in the seventh book/movie.
How do we know that Dumbledore was really Dumbledore, or Severus was really Severus? And the same for everyone else?