What is the common criteria of defining culture of any organisation? Good or Bad?

I have gone through some articles on this, but still looking for some good answers. Major point in other articles was any person before joining any new organisation need to ask about org culture from current employees or ask from people who has recently left the job. My point of view on this is, it's difficult to get the clear picture by this method because the person who is currently working in that organisation will tell you maximum positive points (mostly) about the work culture & on other hand the recently left people, you can contact max two three person & on the basis of those two three people it is very difficult to decide or reach at any conclusion. it's a human nature the person who is still working with that org will start with positive points & the person who has just left the job most probably will start with some negative points. For better answer/clarity on this I am looking for expert views.