How can i distinguish on the map between "perfect vmc conditions" and "no data available"?

In the screenshot there is a rectangular area, that has probably no data. This area, which was in fact not flyable in vmc, is displayed exatly the same way as the area west of the green stripe, which had in fact perfect vmc conditions.

It is not possible in Flyable Conditions to have small islands of no data, due to the way the forecast model works.

I agree the abrupt edge of the coloured section in your screenshot is a little unusual, but not exactly unheard of. If you regularly see such an effect feel free to contact customer support with the exact time you saw it, a screenshot, and the information under What's Here (which I believe shows the model information) and we can ask the data provider to check.

I can see only two possibilities:

a) Weather is changing abruptly over many miles exactly on 48° 00.0 N 

b) There is a bug in the system

From some decades of real life outdoor experience, i feel free to argue, that a) is quite unlikely. 

So you are saying the system has no bugs, which again, based on some decades of IT experience, seems to me to be quite unlikely, too.

Wish you happy debugging ;-)