This is not difficult nowadays, in the modern West - because all of Them, everybody, without any exceptions; are constant, habitual, practised liars. The interesting aspect is what this means; how it affects us.
I find it astonishing when somebody or another in public life is accused of being A Liar. Such are always somebody on the so-called Right - but actually the moderate-Left, because there are zero people in public life who aren't on the Left.
But they All lie, all the time; the people making the accusation, the people reporting it, the people trying to refute it; the people discussing the issue: it is a community of liars. If they were not liars, if they were honest, they would not be in public life.
Lying-to-order is nowadays an absolute pre-requisite of being given a position of high power, wealth or status.
Of course they don't lie equally - some tell bigger lies than others, some tell lies more frequently than others; and of course some are better at concealing their dishonesty than others (e.g. psychopaths, who lack empathy and deeply regard all other people as subjects for manipulation, are usually the most convincing liars; the best con artists).
The thing about honesty is that it is one of the virtues which is actually possible to do for considerable periods of time - and almost anybody can achieve it (if they want to, and if they are prepared to take the consequences). Most people can be honest; at least, to such a level that the occasional lie stands-out (and can therefore, in principle, be acknowledged and/ or repented).
The possibility of sustained 'perfection' does not (for the majority of people) apply to all of the virtues (e.g. love, courage, humility); but it does apply to honesty. We can genuinely aspire to be perfectly honest; and to know whenever we fail to attain that idea.
I know this because when I began as a scientist most of the scientists I met were completely honest about their science. (Those who were not were mostly famous and powerful US scientists, in medical research.) Nowadays, almost no (self-styled) scientists are completely honest - and most are habitually and increasingly dishonest; so I saw the transition, which happened within a generation.
I know the possibility of perfect honesty, and the consequences of abandoning the ideal.
Scientists were probably the last group in The West to adhere to an ethic of honesty; so since science was corrupted we now live in a world of pervasive and escalating dishonesty - in which liars incessantly (and dishonestly) accuse each other of lying.
Some of these liars are so deeply, habitually, professionally dishonest that they do not know the differencetruth and lie. There are all sorts of defence mechanisms and projection at work; some of them are engaged in strategically planned deceptions.
But we don't have the time or the information to distinguish between them, nor or to make quantitative evaluations of the relative degrees to which any specific individual is lying. However, in an ultimate sense, we don't need to know any more than that a person or institution is A Liar - at which point we ought to stop listening to them and ignore what they say; and certainly we should stop trusting them.
Yet the awareness of this situation is minuscule. People don't merely believe the liars, the systematic misleaders, the propagandists and hypers; they actually enforce an ethic that such people must be believed.
Anyone who refuses to believe the pronouncements of known liars in government, the media, the churches or elsewhere is accused of being evasive, of cherry picking - and of course, in a world of liars, they may indeed be engaged in simply reinforcing their own prejudices.
This especially applies to atrocity stories - of war, disaster, crimes, victimisation, fraud, good-works or spitefulness... the basic bread and butter of modern gossip. When some powerful group states that some atrocity has happened, we are expected to believe it or else be regarded as evil - indeed dangerous.
In sum, we know for sure that all official and influential sources are always liars; but belief in their lies has become the usual mechanism by which in-group identity is established.
Belief in lies is our modern initiation ceremony; and this initiation never stops, we repeat it every day, many times per day.
Instead of accepting (for example) some kind of trial of courage or infliction of pain as a coming of age - or else be outcast; we moderns must accept the mental mutilation of believing lies (like pervasive sexism/ racism, CO2 climate change, or the reality of sex change) - Or Else Be Outcast.
This is part of the highly-effective and evil system of totalitarian control.
To know that someone lies, to nonetheless believe those lies, and to know that one is believing lies - is one important reason why almost the entire populations of the UK and other Western nations are literally psychotic.
And this is what enables the evil totalitarian system of virtuality that we all, to a greater or lesser extant, inhabit.