Hmm haven't we discussed this to death?
It is not possible for the police or any entity to tell, just by looking the transactions themselves. In fact, without any actual link to evidence, the transactions can't be identified. If the authority can get one of the addresses/transactions involved, they could get the whole link of addresses that are related but that isn't possible all the time and its rather inaccurate and useless.
It might be true that some criminals might have been using bitcoin in doing illegal or criminal things but we can't say the medium is the reason for all those abhorrent things but instead the person who does it.