Most of my normal tips have already been covered, good advice.
With UK I will do whole panels like doors, trunk lids, fender's etc to avoid the problem. If a roof is featureless, I'll place one of my MF buffing towels as a marker for the area I just applied the coating. For hoods and roofs, creases, other design features like vents, or even a piece of the roof rack or windshield wiper arm, can serve as a visual reference and a "marker" to indicate where your work area.
Even though white is very difficult, using really diffused lighting will help. You'll see a hint of flashing and there should be a slight appearance of wetness compared to the uncoated area for a couple minutes.
My Advice would be to start small and go slow for your first couple of panels. Be hyper observant to what you see and you'll quickly develop your own tricks to being able to see your work.