It is a tendency among many writers to show uncertainty in their work. Look at the following examples:
1) Bob approached his car and found Susan leaning against the passenger side door, presumably waiting for him.
2) He watched her open her lunch pail, probably ready to eat her lunch.
3) The boy was barely able to hold in his laughter.
4) He kicked the ball high up in the air, which seemed to be his method of confusing the opposing team.
5) She fingered her hair-brush, possibly thinking about whether or not she should brush her hair.
In each of the above examples, I used words that make the narrator sound uncertain. You could argue that in a strict third-person POV, the narrator cannot be sure of everything, all of the time, but there are instances where it's better to pretend that they are in order to keep your sentences clean and to the point.
Let's look at example number one:
Bob approached his car and found Susan leaning against the passenger side door, presumably waiting for him.
In this sentence, the "presumably" is not only an adverb, but is also the word that makes this sentence unsure of itself. This is in Bob's POV, so it can be said that Bob might not know what Susan is doing there. Is she waiting for him? Did she mistake his car for her own?
In this instance, however, Susan's actions define her intent. The first assumption a reader will make when Bob finds Susan at his car is that she is waiting to speak with him, so it's not going to hurt the narrative if Bob doesn't show uncertainty over it. Look at the sentence with out the "presumably":
Bob approached his car and found Susan leaning against the passenger side door, waiting for him.
I would probably cut this sentence down even more by not indicating at all that Susan is waiting for him, but cutting the "presumably" helps it along tremendously. Do you see a difference? Do you still think Bob needs to presume anything?
Let's look at example number two:
Bob watched Susan open her lunch pail, probably ready to eat her lunch.
In the above sentence, "probably" is our focus. Again, Susan's actions imply her intent. What is the first thing that comes to your mind when Susan opens her lunch pail? Sure, she could have a thousand different purposes in mind when she opens her lunch pail, but - and I hate to use the word - presumably, she's going to eat her lunch.
So with that in mind, do we really need Bob to suppose that she's going to eat her lunch? Let's look at the sentence without the "probably":
Bob watched Susan open her lunch pail, ready to eat.
This sentence isn't the best of all sentences, but you're already on track to making it clean and concise by removing that seed of presumption and/or doubt.
Let's look at sentence number three:
The boy was barely able to hold in his laughter.
In my opinion, using the word "barely" keeps the boy in a state of inactivity. He's not entirely able to hold in his laughter, but he's not unable either. Above is a prime example of the difference between being direct and indirect.
Using the word "barely" gives the feeling that the writer him/herself is unsure. It creates a more active sentence if the boy is actually doing something, instead of "barely" doing something. Use every opportunity to command your own writing. For instance:
The boy couldn't hold his laughter.
Sounds better, right? You as the writer are labeling the boy's actions directly, instead of leaving him and his laughter in a vague state. It's a small thing, but makes all the difference.
Example number four:
Bob kicked the ball high up in the air, which seemed to be his method of confusing the opposing team.
Here we have "seemed to be". The narrator of this sentence - someone watching Bob play - leaves the possibility that Bob is doing something else by kicking the ball high in the air. But does there need to be that open possibility? Can't we presume that by kicking the ball high, Bob is using his skills to foil the other team? Look at the sentence without the uncertainty:
Bob kicked the ball high up in the air, his method of confusing the opposing team.
Here we have a very direct, clear sentence. Bob kicks the ball high, using his method. The reader gets a better idea of Bob this way as well, since the "seemed to be" allows for Bob to be vague. He could be doing this, but he could also being doing that. I don't know. You never want to say "I don't know" as a writer. Know what you're writing, and who is doing what.
As mentioned, in some cases the narrator of your story will be unable to know something for sure, but weigh the instances where your character can know without becoming an omniscient god; when it doesn't affect the narrative, or the reader's perception of the character and/or POV.
Let's look at the final example:
She fingered her hair-brush, possibly thinking about whether or not she should brush her hair.
That's a lot of words for someone simply touching a hair-brush, right? There's a lot of writerly doubt here with "possibly thinking", and "whether or not". Is she going to brush her hair, or isn't she? Does the reader really need to see your character in doubt over it?
Let's look at an edited version:
She fingered her hair-brush for a moment, and then brushed her hair.
I would go even further with this, and edit it to:
She brushed her hair.
You are removing all doubt on the writer's part, and the narrator's. Remember what the focus of the sentence is: the character brushing her hair. Do you need to up the word-count of your novel or story just to shed light on a character possibly, maybe thinking about doing something? If you want to express the character's constant indecision over things, it might work, but overall, you probably don't need it. Or, to practice what I preach: you don't need it.
There's no "probably" about it.