How to know if the Power supply is from Diesel Generator?

[BobW]

There are two supplies, Mains and Generator
Your flat has ONE meter
Generator electricity is three times the cost of mains electricity

So, when you pay your electric bill, how does the person issuing the bill differentiate between Mains and Generator so that you are charged correctly?

JimB has nailed it. If you can't tell when the DG is running, then with a single meter, how can your residential society tell how much power you consumed with DG power and with regular power? If they can't tell the difference, then they will bill you incorrectly, and it won't matter whether you can tell the difference or not.