How can you distinguish between ‘two’, ‘to’, or ‘too’?

Apart from the context (which is the biggest clue), there is slight, but detectable difference in length:

• to is the shortest. If one were to illustrate it graphically, the phrase To be, or not to be, the word to would be the hyphens:
  

  – —, – / – —

• two is noticeably longer. Thus, the sentence
  Each state is entitled to two senators looks like this:

  — — – √\ – — /\_

• too is as long as two, and essentially sounds the same—but appears either at the end of a sentence, e.g.:
  Itsy, and Bitsy, and Ira, too:

   /`, – /`, – /`, —

  or between two noticeable pauses, e.g.:
  I, too, believe that:

  ˆ, —, √ –

In addition, there is a noticeable trend (particularly in British English) to pronounce the word to as /tuh/—while two and too are always /tu/.

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(Originally written in reply to a question at Quora.com)