Assume

It was while I was still working in Townville and a lot of years ago.  I remember a guy saying, “never assume.”  It is funny that I remember that, but it has become my standard operating procedure.
When I write and state something I always check it out.  I have no desire to put anything down that I have not found to be true for myself. 
It does not matter to me how well meaning or trustworthy a person is, it is important to attempt to find out something for myself before I quote it.  Where possible I do that.  Some things get stated that I have no fool proof way of verifying.  In those instances I will consider the weight of evidence and the reputation of those holding a particular point of view.  I may have no way of verifying something for myself (for example scientific experiments perhaps using expensive equipment I do not have access to).
Even where I cannot prove something or otherwise, I need to be satisfied for myself that what I am saying is correct.  Otherwise I just do not say it.  I find it is better to leave something unsaid rather than go down a path evidence contradicts.
For me a counter argument needs to be evidence based.  Often the proponent simply has a disposition one way rather than proof that that way is correct.
Often in such situations I will remain silent.  There are those closed to any though to the contrary of their own.  Presenting a differing point of view is often a waste of time as nothing presented will change a persons perspective.
Again though I will not assume that is the case.   Each situation needs to be taken on its merits.  Some of life’s biggest surprises are light bulb moments.