When Pressing Shift (for proportion drawings or scaling) turn Arrow or Current used tool to Red (now in InDesign and others it's White for Snap to Page edge).
In addition turn to other colors for other new thinkable functions (i.e Command + Shift = Green, etc) gives a user a reminder on screen to what he is using on keyboard. Could be the colour of the layer-color used or... I leave it to you guys :-)