Hello colleagues, I would like to apologize for the bad English first. My question is next. I am from Bulgaria and here it is very popular to sell fake . How do we know if it is original vag engine oil with specification 504.00 / 507.00 long life 3 5l?

You are going to have to buy from the most trusted seller you can.

Go collect it do not buy online.   Any oil can be in any sealed container that looks like the real thing and not be what it is supposed to be.

Try your skoda/vw/seat/audi dealer.