John's attitude has really annoyed me, mainly because especially after being fooled by Cameron, you'd think he'd be more suspicious of anyone who would randomly strike up conversation with him and seem determined to befriend him.
Add to that Riley constantly saying things like "you're full of carrots and apples" or something like that to John, which are exactly the same strange attempts at human sayings that the terminators keep coming out with when they're trying to sound normal. His immediate trust of her has been so irritating to watch I actually think it was a big reason I disliked Riley (I'm not a shipper so her blocking John/Cameron didn't bother me)

But the fact is, no matter how annoying I found that it could be explained by John being traumatised and desperately needing a connection to someone he can believe is normal. That's a bad explanation but at least it's an explanation.
Derek has known Jesse a while, so he knows what she acts like when she's not up to something. He knew her intimately plus he was the most suspicious character on the show. He spent years at war, he's not the type of character to fall for this type of thing and the only explanation I can see is "Jesse is hot."

Derek has more experience than John, he should know better. Although I wonder if he's ever really known Jesse. I think she's the kind of person that always keeps up a front and never lets anyone see the real her. She always plays people. I think she's played him from day one.
Like candrani said, John's desperate for normalcy. I think that's blinded him somewhat. Cameron "checked" Riley out, so he knows she's not a robot, but I am confused why Sarah at least doesn't seem to have done a background check on Riley. Although, if she had I'm sure Riley would have checked out. That stuff can be faked obviously. But John has acted like a brat when it comes to Riley. It's not her I dislike, it's the way John's acted since she came into his life.

I wonder if Jesse knew Derek was John's uncle when she got close to him in the first place. I know it was meant to be a secret so it's not that likely, but if people have been plotting against John it makes sense that they would look into him & his past. Maybe that's why she got close to him, to have a connection to John.

Concerning Riley, I wonder if John will be as interested in her now she isn't his connection to a normal life. I always thought the main basis of the relationship was him rebelling from his fate, so now she doesn't represent that I wonder if he'll want her as much.

I suppose he might, because she would still be a rebellion from Sarah, who wll be furious when she finds out what Riley has been up to and will be even more against the relationship. Also, I guess just because his initial reason to be with her was to rebel that doesn't mean he hasn't come to care for her. I'll be interested to see how his feelings for her change one way or the other after finding out the truth.

Yes, it will be interesting to see how he's going to react to Riley's secret. I hope he doesn't turn his back on her because I view her as a victim in all this.

Derek has been a world class idiot when it comes to Jesse. But she is much better at manipulation than Riley and has managed to explain away doubts that Derek had.

Riley has been acting twice as suspiciously as Jesse and John doesn't appear to have had any doubts at all(edit: John sure showed me, lol). At least when he thought Jesse was up to something Derek called her on it. John seemed to just accept every bizarre thing Riley did without once making her explain herself.

I agree with Fanfly that Riley is a victim, she is in a very difficult position. Though I have to say (and I'm going to sound kind of bitchy) I think the actress playing her makes her quite hard to like. I'm still annoyed by Riley, but I don't ship John/Cameron and I feel very sympathetic to her situation so I don't know why she bugs me. I have liked characters who have done worse in easier situations.

Well, after seeing episode 19 I've changed my answer :)

Apparently John isn't as gullible as he seemed.

John did finally figure it out.