Truth: Noun. That which most easily appeals to the fears, hopes, and prejudices of the hearer; in essence, a lie. Lie: Noun. That which causes its utterer to be reviled; in essence, the truth.
Deceit: Noun. Example: A dog rolls on the carcass of a dead possum in order to deceive other animals regarding his identity as a dog (whereas) a man lolls in the scents of church in order to deceive himself regarding his identity as a thief, an adulterer, and a liar (the difference is that the dog does not first have to endure being bored by the possum).
The thing about truth and deceptions is that they are always of a certain kind. If only you could recognize them before they arrive... Here are a few hints.
First, recognize honest intent, whether expressed as the plain truth or a technical falsehood. Possibilities include:
Metaphoric Truth: 'I am a child at heart', 'The Earth is our mother', 'All men are brothers' -- all are known falsehoods spoken without intent to deceive. My father is not my brother, and my mother does not weigh 6.5 hexillion tons, but no one would take such statements literally -- well, almost no one.
Second, learn the myriad shades of deception, purely for self-defense, of course.
Big Lie: Repeating a colossal (and frequently blatant) untruth, as loud and as often as you can, will make it easier to believe -- because no one would conceive that anyone would distort the truth so boldly and outrageously. Attributed to Lenin as "a lie repeated often enough becomes truth".
Cover Identity Anomaly: Missing some basic piece of information about someone you're pretending to be, often forcing you to make up something on the spot.
False Reassurance, a lie that is the truth in its Exact Words. A guard at a railroad track back in the 1920s was testifying that he was swinging his lamp when the automobile crashed into the train at 3 in the morning. It was determined later that while he was swinging his lamp to warn oncoming traffic, the lamp was not lit at the time.
Half Truth: A statement that is technically true, but extremely misleading if you don't have all the information.
Ignorance Is Bliss: A character or characters are told that what they are doing is something completely harmless or beneficial when it's actually dangerous or evil.
Outright Lie: 'I am trustworthy', 'The Earth is mine to sell' -- both are falsehoods knowingly spoken with intent to deceive. This is the trademark of the Con Man and the schemer, resorted to by anyone covering up a Big Secret. However subtle they may be, Pull the Thread and they collapse (which is why the liar often takes Refuge in Audacity, to keep someone from pulling the thread).
Sarcastic Confession: A truth that will not be believed. Often used to support the Masquerade, this both deflects attention from the truth now and makes one less willing to accept the truth later.
Truth Twister: "Looks are only skin-deep," "The Earth is doomed," "No one who has drunk my tonic has ever died" - all carefully selected truths, intended to deceive, the hallmark of advertisers and politicians. Expect anyone unable to lie whether due to magic, Applied Phlebotinum, or just being a Manipulative Bastard type, to be good at this.
Unusually Uninteresting Name: This innocent sounding group are not a secret organisation of any sort, you won't even question their involvement or even notice their existence. The name is likely to be technically correct though.
You Didn't Ask: when a character simply doesn't tell/mention something that's particularly large, hard to avoid in conversation, or just really should have been mentioned.
You Said You Would Let Them Go: A villain lies to someone to get them to go along with a plan that will ultimately hurt those that they care about, or otherwise makes a promise that he has no intention of keeping. Often involves use of loopholes.