Antonym for lying?

There is truthing, to truth (as in the lyrics) but it is nonstandard or a neologism.

Wiktionary has three definitions for the verb truth where the transitive usage is obsolete:

1. (obsolete, transitive) To assert as true; to declare; to speak truthfully.
2. To make exact; to correct for inaccuracy.
3. (nonstandard, intransitive) To tell the truth.

OED definition and the latest citation:

intransitive. U.S. colloquial and regional. To tell the truth.

1993     J. Womack Random Acts of Senseless Violence 237     She was lying not truthing

Etymonline says that:

English and most other IE languages do not have a primary verb for "speak the truth," as a contrast to lie (v.).

You can see examples in literature and technical books also if you check Google Books. Some examples have different senses and the gerund form of the word seems to be more common. I've excluded the word ground from the search as there is the technical term "ground truthing" also.

Furthermore, Wikipedia includes this note about verbification under Conversion article:

Verbification may have a bad reputation with some English users because it is such a potent source of neologisms.

Although some neologistic products of verbification may meet considerable opposition from prescriptivist authorities, they are very common in colloquial speech, particularly specialized jargon, where words are needed to describe common actions or experiences.

Level (with) is a verb that can be considered but it is not an exact antonym. OED definition:

To be honest or truthful; to tell the truth, speak frankly, behave honestly or deal straightforwardly (with). slang (originally U.S.).

Other than that, aver might be a close contender but it is mainly used in legal context. MW definition:

to verify or prove to be true in pleading a cause

That's an interesting question. I don't think I've ever come across a pure single-word antonym for 'liar' or 'to lie'. The closest I can give are near-antonyms which serve more to corroborate with existing truths, like attest, validate or testify.

A little bit of research seems to indicate that there is in fact no pure antonym, and the only option is to use a verb phrase instead:

"I lied," is the verb;
"I told a lie," is the verb phrase.

So the antonymous verb phrase would be "I told the truth."

Can't seem to come up with anything better than that right now. I briefly entertained the possibility of an antonym to 'prevaricate' like 'varicate' but apparently that's an adjective pertaining to medical research, so no dice.

I'm going to suggest confess.

1. To admit to the truth, particularly in the context of sins or crimes committed.

It would fit your lyric well. "You keep lying, when you oughta be confessin'"

I take it you want an antonym for the verb lying. I don't think a single word antonym exists yet in English. But I think it might be in the making.

Wikitionary has an entry, albeit a short one:

Verb: truthing, Present participle of truth.

The Urban Dictionary, however, has an entry for truthing: the act of telling the truth.

It also has one for creative truthing: "When you present the truth in a very unflattering way as a means to deter another person's romantic interest in you (most useful in online dating)."

I was grossly unimpressed when I met Joe for a coffee date. I wasn't lying when I told him that I wanted to get married, have two kids and be a stay at home mom. I was just creative truthing.

Full Metal Mommy, a mommy blogsite, dispels the lies and myths about potty training in Potty Truthing.

Another website, The Human Potential Center, has an article titled TRUTHING AND TRUSTING - An honest look at the effects of dishonesty

An article on Wikileaks: Truthing People into Peace

There is a book about relationships that emphasizes truthing:

“Truthing” is the word we have been using in this chapter to designate simplicity, clarity, honesty and humility in communication. Truthing seeks simplicity..., clarity..., honesty...

It gets a substantial number of hits on Google (- ground, as ground truthing is a method of data collecting in the field).