How would I create a fruit ninja knife effect?

A couple of general comments.

It would be better to show the fruit inner surface not just every fruit side on. It would also be better to start with a higher resolution image.

However starting with your image - I used layers to build up an example of what you could do:

Draw the sword with the pen tool set to shape and fill the shape with a gray gradient.

Then use layer styles to add a bevel edge and inner glow.

Copy the fruit - turn upside down and clip to the sword layer so that they are only visible in the sword and appear to reflect

Copy the sword and use liquify to drag the back edge and a mask to fade it - this will suggest some movement

Add some faded strokes with a white pen and some "cross" highlights with the brush

Copy the top half of your image and place it above the sword so that it appears to cut

Finally a few water droplets - just hard brush strokes on an empty layer with the layer style set to give two inner shadows (light and dark) and a radial blur using the zoom setting to suggest movement.

Here are the layers I used to give an example.

If you want detail on any particular layer let me know.

Above all - have fun !

Dave