How can a script know whether a user can input or not?

Is there a way a bash script (or whatever language) can determine whether its being run in a headless way? I want to know whether a user can input or not. If they can, I am going to ask them something.

Solution

From man bash:

An  interactive  shell  is  one  started without non-option arguments and without the -c option whose standard input and
       error are both connected to terminals (as determined by isatty(3)), or one started with the -i option.  PS1 is  set  and
       $- includes i if bash is interactive, allowing a shell script or a startup file to test this state.

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