I've tried in full sun and total darkness, and everything in between, and there's just no way to see what the coolant level is in the reservoir bottle without taking out the battery, and maybe not even then. I don't know how they check it at the dealers.
Anyway, I thought it might work to use the rubber hose that goes down into the bottle as a dipstick. If the hose extended three inches into the coolant when the engine is cold, does that sound like a reasonable fill level? As the engine heats up, the level would rise, and the only danger is that it would end up too high. (Although I don't see that that would actually hurt anything.)
And three inches would allow for a fair amount of time between checks without the hose ever being out of the coolant.
What do you think? Three inches? More? Less? I don't suppose there's an official coolant dipstick designed for this car. Nah.