That's one way to know but what if a person bump a thread by pressing the bump link/button and do not post ?
What you need to know
3. If you've been spam-bumping, knock it off, since it's not going to work anymore.
I think that explains it unless a user does a mini bump which is kind of a hassle if the user is a regular poster there.
Mini bumps
If you haven't posted in a topic in the last 7 days, posting in it adds 1% of your base bump power to the topic's bump score. Deleted posts count against the 7-day limit, but not the topic's bump score.
The reason for knowing is what if a single person is bumping a thread.
Even if he is the only one bumping it his bump power decrease over time and it takes diligence to do super bumps because it is done only once per 24 hours. The real question is "what if there are several accounts used to bump the thread?" i.e for promotional purposes. I don't know who really can see those who bump the thread if mods and GM or only the admins can see it.