Strings is a program that prints out any ASCII or Unicode strings in the input file. Forensic examiners can use strings to get a sense of the functionality of an unknown program. User prompts, error messages, and status messages can give hints, but should not be used as proof or lack or any functionality.
Most Linux distributions and other UNIX-like operating systems have a strings program included.
There is a Windows version of strings by Mark Russinovich. Note that the Windows version prints an output header and searches for both ASCII and Unicode strings by default.