It’s often very useful while editing your keyframes to be able to see previous and even later key frames on the canvas at the same time as the current keyframe you’re editing. Spriter allows you to do either or both.  Here’s how:
Using the Onion Skin Featue
To enable the onion skin mode for seeing previous key-frames.. just right click and drag on the left hand side of the red line that denotes the currently selected time in the main timeline.  A red gradient will appear between your mouse pointer location and the red line that denotes current time.  The length of the gradient controls how strongly previous keyframes will be displayed on the canvas.
To enable the onion skin mode for seeing later key-frames.. just right click and drag on the right hand side of the red line that denotes the currently selected time in the main timeline.  A green gradient will appear between your mouse pointer location and the red line that denotes current time.  The length of the gradient controls how strongly later keyframes will be displayed on the canvas.