How to determine if a process has GUI or not?

Question

How to determine if a process has responsive GUI or rather it's a background application? May be there are some API function calls that are necessarily made?

(The answer doesn't necessarily has to involve API calls)

Answer 1

The closest Windows API call to deduce this information would be GetGUIThreadInfo . Note the following paragraph from the Remarks section:

If the specified thread does not exist or have an input queue , the function will fail.

A thread with an input queue can potentially own windows. Note that a thread with an input queue is not required to create windows. Note also, that a non-GUI-thread can be propagated to a GUI-thread at any point.

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Note: Even though executable images specify the subsystem , under which they run (eg CONSOLE or WINDOWS ), this cannot be used to distinguish between a process with a GUI and a 'background process'. A process running in the CONSOLE subsystem can create a GUI just as well.