Method to find out if Microcontroller IC is counterfeit
I'm curious to know if it consistant among all ATMega chips, or is it
a coincidence? Anyway, does my measuring make any sense?

In this case it's probably just coincidence. All your resistance measurements are so high that they only represent leakage, which could vary for a variety of reasons even in genuine chips (which we can see from your numbers).
Resistance measurements can be useful as a first step in determining whether suspect chips are likely to be at least similar to what is marked on them - but not the way you are doing it. A better way is to put the meter on 'diode test' and try to measure the presence of protection diodes. This can help to identify Vcc and Ground pins, and whether other pins are at least connected to something inside the IC.
CMOS ICs usually have protection diodes on inputs and parasitic diodes on outputs, whose forward voltage drop can be measured between I/O pins and Vcc/Ground with 'reverse' polarity. You can do the same to determine which pins are Vcc and Ground, where the 'diode test' voltage is usually significantly lower. You can also compare the results to a known good chip (if you have one) and be suspicious of any large variations. Do not make any assumptions regarding pin identification until this test is completed!

What would be the next step to test those chips? Thanks!

Once you have verified that Vcc and Ground are on the correct pins, power up the chip and check that other pins are inputs (high impedance) or outputs (stuck high or low, or pulsing) according to what the datasheet says they should be at startup. Then create a circuit that tests the chip's functions. For most MCUs connecting them to a programmer and reading the device ID should be enough.

I purchased various ATMega uC chips from Aliexpress. I know you never
know what you get that way (usually what you pay for!)

If you want to know what you are getting then buy from reputable suppliers. If you are  manufacturing a commercial product any price difference will be outweighed by the greater confidence, and for small quantities it usually doesn't matter (and the time wasted trying to get a fake chip working is not worth the cost 'saving').
When purchasing new ICs I only deal with companies such as Farnell, Mouser and Digikey, who have impeccable reputations and guarantee their supply chains. However sometimes I need a 'vintage' chip that is no longer in production, and then I am forced to buy from eBay. I usually deal with a small number of vendors who supply genuine surplus and recycled chips. Others have suspiciously low prices and parts that appear to have been scraped and remarked. With these you never know what you will get.
Occasionally I might get a remarked chip which is completely different because it had a 'similar' part number by coincidence, and the refurbishers were too lazy or ignorant to separate it from the rest. However most of the time they are least roughly equivalent to what is marked on them, but might be NMOS instead of CMOS (which is easily detected by measuring supply current) or a lower spec part. This is unfortunate because I care more about having a chip with known specs than a 'premium' fake, and even prefer chips that look shabby because then I know they are genuine pulls. Vendors of 'fake' chips should realize that they while they might con a few suckers by remarking them, they lose sales overall - so 'prettying up' chips to make them more attractive is a waste of time and effort.
Another way to get cheap parts with higher confidence is to buy complete modules such as Arduino clones. Millions of these are sold on eBay and through retail electronics shops, so if they didn't work the manufacturers would quickly get into trouble. Whether they are 'genuine' Atmel/Microchip MCUs or not is irrelevant if they work properly. I have purchased hundreds of these and other modules, and not had a single dud.