Can a person be taken seriously if they don't say what's on their mind? The short answer is no, but let's analyze the why people trust dishonest people more.
First thing people often look for is language, and there is even a study done to prove the points in here. Often bad language is thought to only be a specific subset of words, and this is what messes it all up.
If a person is more worried about avoiding that specific subset, then they are more likely to worry about social opinion over fact. Foul language includes using any word in a different manner than the socially accepted definition, so sorry you prudes but "fuck" is less foul than saying "fudge" instead.
We have been conditioning people to accept thought control and police, by doing so we have created the perfect set of cues to detect honesty. When a person who is staring at someone for any reason turns away, you can be certain they are less likely to be honest when asking them an opinion.
There is one exception, when trying to look coy so as to pique the interest of someone else, in which case they are likely to be very open and honest. The coy look is often associated with those who are excessively sexual, the term slut being used for them.
However, the coy look is not about sexual activity, it's about getting your attention for any reason, though sexual activity is jot a bad thing, without it you'd not be able to read this. Nervousness when called some of the shaming terms is another sign of dishonesty.
An honest straight person is likely to play off being called gay as a joke, no shame, no embarrassment. If a person called a shaming term gets angry or upset then they are hiding something, usually the thing the term implies, this means they are not honest with themselves.
Now when asked an opinion if they don't at least try to make an effort to analyze before answering they are probably not being honest. A negative opinion is always an honest one, but a positive one can only be honest if you take the time to form the opinion, otherwise it's a platitude.
Violent people can be trusted, they are honest, but the fact that they're violent is a good reason to not befriend them. Yes, pretty much anything looked down on by society is a good cue, if they exhibit the behavior not accepted by society then they are probably being honest.
Online honesty become compulsory, so much that we often share more about our thoughts than we intend to. When you learn to embrace this honesty you become something many people envy, you reach a level of awesomeness that is often exclusive to celebrities.
Now you're probably saying that not all speech should be allowed, but that would remove our greatest tool ever. When we see racism, for example, we can identify the pattern that breeds it then address the problem at the root, and that's how you kill all weeds.
For racism we see it brewing most in uneducated, religious, and heavy gun ownership areas. So we can address those issues and reduce racism, eventually we can force racism into the dark region of our specie's past, the place the Crusades and slavery reside.
So let's stop being such pussies and spread honesty across the globe, make our species tigers instead.
What? What other pussy would a cat person be talking about?