One way I determine if its an actual person is if the withdraw ends in *.98, because *.02 is the fee Bittrex charges for withdraws. So if somebody withdraws 1000 Coins, 999.98 will get deposited.
That'll never work because it assumes that everyone withdraws a whole number of coins.
You'd have to figure out what the addresses are of the exchange wallets, and look for transactions to/from them. But this will also pick up people who are simply using exchanges as a way to obfuscate/launder coins via off-chain transactions.
Most people do not even have a whole number of bitcoin. The only real group of people that would have a whole number of bitcoin would be solo miners who do not include any TX in their blocks