The clues are there, some more overt than others.

Littlefinger may not have explicitly stated "it was all my doing!", but he did make clear that he purchased the necklace and gave it to Dontos, and further demonstrated how the jewels were breakable. You can also see at the moment, and in the wedding episode, there's a jewel missing, which helps connect the necklace to Joffrey's death.

Ultimately, after episode three, your options are:

A) Littlefinger planned everything in collusion with Ollenna Tyrell and their dim, drunken accomplice.

or

B) You don't trust anything you've seen and have no idea who poisoned Joffrey, as there's no evidence pointing to anyone else.