So I have a bank account in which I've deposited $100 in $20 notes.
Let's say I write a cheque for $20 - How do I know which of my various $20 notes was used?
To answer your original question, you don't, and if you think you need to, you're probably doing something wrong.
i was *thinking* to test this 'famous' double your bitcoin on .onion and they need it
but now when i see replies. i assume this stuff is pure scam and i shouldnt riks even pennies with it