Umm - but that would also be true when the phototransistor is fried ON at all times, wouldn't it?

-Guess you'll have to provide a continous positive feed to R1, (when the LED will be OFF if the coupler's OK)
and the pushbutton only to pull some current thru the optocoupler's LED (turning the D1 ON).
Having the PB to supply everything will not test for a faulty, continously ON phototransistor.

Cheers