Re: they just left after posting because they were attacked and harassed.
Yes, these things are easy to track if you know how.
Click on a person's user name and it will show you all of their previous posts. You can go down the list and read their posts to see if they are really here being helpful.
You can also often see where a user suddenly stops posting and a day or two later, you will see some new user posting with almost the exact same temperament, grammar, and attitude. Then you can look and see who has been here for many years, with many helpful posts on many topics.
When someone suddenly shows up on forums and starts attacking a person or a product or even an idea, you know that they have come with an agenda and are not really here to help.
If they respond vehemently to a testimonial, you definitely know that they are not here to help but that they have an agenda and are probably being paid to pursue that agenda.