You can see how the water sheets off, but beading is a clear indicator.

I usually watch how the water beads up. If the bead is close to spherical, the wax is still on with a significant amount of thickness, where as when it starts to look uneven with more of the water drop adhering to the surface it's time to schedule a rewax.

I usually mist-spray the car after a wash but before drying to inspect the panels which require attention on days which I'm strapped for time. Usually areas which have are prone to wind drag and primarily exposed to the sun require rewaxing first, such as the left and right mirrors and bottom half of the bonnet.

Not much of a difference, there was a test done at Autogeek where a forumer laid down wax optimized for different colors and categorized them with different multiple layers for comparison. To the naked eye there's almost no difference, but once digital image alteration came into play the colors were clearly visible.

IMO, not worth it. You're better off with a 'neutral' wax.

Had some fun with OCW today. Now I know you don't put them on your windscreens, but I threw caution to the wind and gave it a nice spray and buff after washing and drying.

It was difficult to to buff off, even with priming the MF and the surface clung to the cloth. Not only that, it would streak but it was minimal - it was a test, so it didn't matter to me but never I would try OCW on it again with the amount of time spent to get it right... this is what happens when you don't follow advice

Now I was in KL today when the sky poured - beads, glorious beads! The rain clumped together and flew off easily at low speeds, even when cornering - shows how hydrophobic OCW can be on your paint.

Now if I can get my hands on a good bottle of Opti Seal it would be my next test. Many people have reported good results, so we shall see

Just got a 100ml bottle of HydrO2 since it's Black Friday. Seeming from how easy it is, can't wait to test it out.

There needs to be a distinction between CLEANER wax and pure waxes...

Usually you can find out which one is which from the instructions.