to trick or not to trick ?

On Halloween, the opposite of "trick" is "treat".

To be honest, he said frankly, I'm an open book, but I can't think of a single word that does it on its own.

What would be that opposite state of tricking someone?
Being honest. Being truthful.
I have heard the word undeceived, but I believe in adjectival form. I'm not sure it's a verb. And even if it is, I don't think it's the exact opposite of deceive, in the same way that being truthful is the opposite.

There's always inform, but that might be missing the point slightly.

Maybe there isn't a word for this. After all when we tell someone something, or when we state something, it is understood that we're not lying unless otherwise noted. Merriam-Webster offers "enlighten" as an antonym for deceive, but I am not sure that that is a precise antonym. To me "enlighten" implies leading someone from ignorance into knowledge (sounds pretty noble that way!), again with an implicit veracity.

Perhaps the Ferengi on Star Trek the Next Generation would have a special word for telling the truth.

"I'm not going to trick you; I'm going to _____________ you."
untrick, undeceive, enlighten

I wondering now whether there is a transitive verb that is the antonym of "to trick, to fool, to deceive."
To assure.

I think Jackie's offering of to assure is much closer to an antonym of to deceive, although it is possible to outwardly assure someone of something and yet be lying.

...although it is possible to outwardly assure someone of something and yet be lying.

Surely, Alex, you're not suggesting that someone may smile and smile and be a villain? Nah, never happen!

Theresa, I don't think there is a simple verb which fits your bill. The concept of non-trickery must be so commonplace that no one ever needed such a word!

Except, of course, the Ferengi, as ?dxb suggested above ...

- Pfranz

And that suspicion--that there isn't such a verb--is why I started the thread.

I find it very interesting that we do have this concept of directly tricking someone, but when we do assure someone of the truth, to use Jackie's verb, we don't have a single transitive verb that does the trick--at least one I can think of.

We trick each other, we fool each other, we deceive each other--all very direct actions. All transitive verbs.

But where is that verb that clearly shows in its transitiveness that we are leveling with each other? See? There's that bloody preposition. You'd think we'd have a verb that would be the eqivalent of "leveling with"--but perhaps we don't. And I find that to be odd. And very interesting. If true.