How can I test whether oil is leaking into cylinder combustion chambers and getting burnt in case of oil level loss? As opposed to, for example, leaking through the oil pan or the timing chain cover.
You can test your rings and head gasket with a leak down tester
Checking your spark plugs as shown in @Zaid's post is the first order of business. You will see it on the plugs.
Now, if you do see an oil fouled plug, you will need to identify the source of the oil.
Possible Sources of Oil
Intake valve guide seal
Any of the above can be a source of oil into your engine where it can be burned.
To test the integrity of the combustion chamber you can use a leak down tester. A leak down tester will pressurize your combustion chambers and allow you to listen at your intake and exhaust valves for leaks as well as your crankcase ventilation for head gasket and ring leaks.
Here is post regarding how to use a leak down tester.