Re: "...(is there a way to tell?)..."
I'm a bit weak on the details, but I've read that it's a PUBLIC ledger.
Well yes, but that does not mean that it is traceable to a particular person. If you spend any portion of a bitcoin your "change" will be from a completely different place. There are also sites that deliberately "mix" your bitcoin to prevent it being traced. It is somewhat similar to the serial number on a banknote - those could perhaps be used to determine who originally withdrew a particular £20 from a bank or ATM, but if that person used it to buy an item for £1, the money he received in change will no longer be traceable to him.