which

This tool locates executable files on the PATH. And it does a lot more. It can be used to determine which executable Windows will call, when multiple copies exist. Usage:

usage: which.py [options]

options:
  -h, --help    show this help message and exit
  -x EXTENSION  search for EXTENSION
  -d DIRECTORY  include DIRECTORY
  -r            search directories recursive
  -e            C-style string formatting in quotes
  -b            break after the first result

To find notepad on the path, try:

c:> which notepad

To locate an include file, you can try the following:

c:> which windows.h -d "%INCLUDE%"

Note: You need quotes around %INCLUDE% because the environment variable is likely to include spaces.