How do control buttons know they've been clicked?

I have written an application in c++ visual studio 6 that looks for the WM_LBUTTONDOWN message to carry out a certain task. This works fine except when using a touchscreen. The Wm_lbuttondown or any other button message is not sent when the screen is touched. However, the control buttons still operate normally, so they know they are being clicked. I would like to know how the buttons detect the touch. I would like to detect when the screen is touched in a similar way to how I detect a mouse click. Is there a different WM I can use other than WM_LBUTTONDOWN? My old version of visual studio doesn't seem to recognise WM_TOUCH so I haven't been able to try that.