It would make sense since she was the first one giving him an opportunity and now she would be the one who finds out that he's on the -A team.

I don't know why, but I think it's her.
I just can't see Spencer or Emily finding out...

I think it's most likely that spencer finds out because she is closest to him

I dont know why but Hanna comes to my mind first and I think Spencer will be the last one to find out.

i think she's the one least involved with him so he'd be less careful around her, also she'd have to break it to spencer and emily
i agree with bbiatch that spencer will find out last, maybe the other three will come together to tell her like they did with paige

I hope Emily will. I ship Emily x Toby and they're so cute together + they're friends... at least they were in first and second series.

I think either Spencer of Em would find out the truth.

Em was the first one to trust him and the first one to be betrayed. I'm totally hoping that it happens like this! It would make much sense.
I agree with some people: Spence should be the last one to discover it, or atleast after Em, it would make such an interesting storyline!

Hanna was the first to find out about Emily being gay, Aria hooking up with a teacher, she figured out Kate photoshopped the picture and was the only one to confront Jenna about faking her blindness, the other girls just followed suit. She tends to have a way of finding everything out, along with Spencer. But there's no way Spencer would believe Toby was A, so..

Hanna i think :)