How do you find the document number on a visa?

Issued visas have a code that is a combination of alphanumeric numbers. Usually, this is preceded by the country initials, and is written (1) on the upper right side or on the border of the visa, (2) in a color different from the rest of the visa text.

Example:

TH678901 = This means the visa was issued in the Thailand consular office, visa number is 678901