I though Poland has left hand driven vehicles.
About the AC. You can also check proper freon level through the glass window located about the filter/drier. There cannot be bubbles when the AC is running, and as soon as AC compressor turns off, you should see bubbles.
^^^^^^^ I would argue you can get 'proper' level/charge this way. It's a remedial way (sans gauges) to tell if you have enough liquid refrigerant (or any at all) in the system. But to get the 'proper/optimal' amount you will need to have a set of gauges.
The BEST way to charge a system is to 'weigh in' the refrigerant, but this requires a complete discharge. Often times a system (slow leak) only needs to be topped off. So a set of manifold gauges are used to monitor both the high and low side pressures. This is the second most accurate way. The third way to charge a system (but the least accurate by a long shot) is by watching the sight glass.
What you are looking for is a clear, steady stream of liquid refrigerant (engine, a/c system running). But you have to walk a fine-line. An overcharged system will most certainly display a clear, steady stream BUT your high side pressure can be off the charts. Also, there are certain conditions (due to ambient temps) that will cause 'flash-gas' to appear as bubbles in the sight glass. So...naturally the uninitiated will add refrigerant, quite possibly over charging it. Also, air in the system will always appear as bubbles.
Using BOTH a set of gauges AND observing the sight glass when recharging (or topping off) is not a bad way to do it. Since you can monitor when acceptable levels of pressure and disappearance of bubbles coincide. This will put you on the 'cusp' of peak performance (for the ambient temps at the time).
Being undercharged (gas-liquid mix) won't harm anything...it will just result in net loss of cooling capacity and high vent temps. But being overcharged (significantly) is not good for your compressor and also results in poor vent temps.
There are already many variables to consider when adding/charging refrigerant into a system without limiting yourself to a 'single' mode of feedback (sight glass). Its a handy thing to have when you are way out in BFE and have no gauges. But if you have gauges or can get access to them...use them.
