Kung-fu translates to “any study, learning, or practice that requires patience, energy, and time to complete” (Wikipedia), but in the West it tends to be an umbrella term for all Chinese Martial Arts. Writing a style guide for all of Kung-fu is a bit misleading as there are hundreds, if not thousands, of different styles and variations. Every family and lineage has its own name and stylistic differences.
As a sweeping generalization, look for hand positions*, arm and leg extension, circular motion (as opposed to linear), levels (stances that stand upright, stances that are very close to the ground, coupled sometimes with leaping and fighting from the ground), fluidity and rhythm. Often there will be mnemonic devices built in, as in the various animal-based styles. Remember these are vast generalizations! If you are creating choreography, be sure to research a specific style and discover its specific qualities.
For an example of unarmed Kung-fu against kickboxing, here’s Jackie Chan fighting Benny “the Jet” Urquidez in Wheels On Meals.
And an example of armed Kung-Fu from The Legend of Drunken Master.
*a very small sample of hand positions: