Is Haki visible to other characters?
Both Kenbunshoku and Haoshoku are not visible at all, since, well it was not made as something that is visually needed to the reader, since they're basically perception and sheer will power respectively.

Busoshoku, however, can be seen both by the user and others alike, though not all the time, since it acts like an "invisible armor" or coating, in a sense. At that point, it is not seen, which could be demonstrated by someone like Zoro, for example. But, when in is concentrated heavily on certain areas of the users body (i/e: Luffy or Vergo) or to a weapon of theirs (i/e: Vergo or Smoker), it hardens said part (or extension of the users body) and turns it black, making it "denser".    

(As for Vergo's extension for to his bamboo: Chapter 672, page 4 Smoker: Chapter 690, page 8.

For a use of Buso, without demonstrating it physically: Chapter 512, page 12, among many others. Zoro vs Monet for instance.)

Hope this helps.