Well I had mentioned this a few pages back but I was trying to decide whether Solas had truly lied to us? If you asked him a question, I believe he was honest with his answers. If he couldn't give you the truth, he would hide it behind a clever comment or question. He omitted to tell us the truth and until the end before he was ready to confess you could see his conflict.
I guess it depends how you see it. He would slip now and then and when you would point out, he would immediately mention the Fade. I thought Solas was honest in his feelings and the answers he would give you.
Now whether he would answer if he was the Dread Wolf or not, I think he would tell you. How would Lavellan figure that out if she hadn't drink from the Well though? Poor Lavellan.
If he doesn't lie directly, he lies by omission. (As in, omits the facts.)
When you first talk to him in Haven, he mentions that he thinks an artifact might have caused the explosion that caused the Breach, but omits to tell you he know exactly what kind of artifact it is.
When the Herald is coronated the Inquisitor, he pulls you aside and mentions that he knows that mysterious orb Corypheus used when he attacked Haven was an elven artifact and he wants you to help retrieve it unharmed. Again, omits that he knows because the orb was his and he was the one who gave it to Corypheus.
Then when other characters ask him how he knows so much, he tells clever "technically true"ths or {rom a certain point of view" truths, but never outright lies - unless you count when he talks to Blackwall of Val Royeaux. Solas claims he saw it as a small settlement of market tents in the mud in the Fade, though it's likely he actually saw it in person. (Including the smell.)
I think the only time he lies directly is if the Inquisitor talks to him right after the Orlesian Ball, where he mentions how much he missed the dangers of court intrigue. If you ask him directly "When were you at court?" He immediately starts to backpedal, and protest that an elven apostate would never be allowed in court, of course; he only saw it in the Fade. Interesting, this might be the only question you earn disapproval of in the game, and likely because you've forced him to lie directly to maintain his cover.
I guess, like you said, it's up to the player to determine how much his omission of facts count as actual lying or not. Since he seems pretty open about his emotions, opinions, worldviews and experiences aside from giving concrete names, dates and ages, I guess it's easier to stomach than "Yes, I'm Blackwall" or "Yes, I'm a Grey Warden."
As for how he would react if the Inquisitor asked point-blank... I think it depends on the context or timing. He might protest fumblingly like he did about when he was at court. More likely, I think it would be "an armor-piercing question." I think his face would give him away, and then the ball would be in the Inquisitor's court.
Just my view.