Determining the difference between fact and opinion can be challenging. The most basic difference is that facts can be proven whereas opinions cannot. When attempting to distinguish between the two, consider whether any objective proof exists to support the statement. If so, it is most likely a fact. If not, it is most likely an opinion.
For example, it is a fact that some apples are red and some are green. There is ample documentation supporting the presence of both types of apple. Someone can visit a grocery store, apple orchard or farmer's market and physically see that both types of apples exist. No matter how a particular individual feels about either color of apple, there is no question that some apples are green while others are red. The color of apples is objective, meaning that no matter how many people look at them, there will still be red apples and green apples.
If a person states that red apples taste better than green apples, he is expressing an opinion. The statement is subjective, because it is based on preference. Another person might prefer green apples, while a third might like both equally. No empirical data or proof exists to support the opinion that red apples taste better.
Sometimes fact and opinion can be confusing. For example, a man might say that he likes red apples better than green ones. This sounds like an opinion because he is expressing a preference. The statement is actually a fact, however, because he is stating a provable piece of information. An objective observer could verify that the man does, indeed, prefer red apples.
Confusion between fact and opinion can also arise when an expert states an opinion as though it is fact. For example, a cookbook might say that green apples are better for pie-making. This sounds like a fact, but is actually an opinion, because the term "better" is subjective. The cookbook author may have meant that green apples have a lower water content than red apples and therefore keep the crust from becoming soggy. This statement would have been a fact because a green apple's water content and effect on a pie crust can be observed, measured and proven.
In the author's opinion, a firmer crust is better, so he states that green apples are better. Someone else may prefer a softer crust and may therefore want to use red apples instead. Another person may not like green apples at all, so red apples would be better for him, regardless of the water content or its effect on the crust. As this example indicates, the difference between fact and opinion can be quite subtle and may depend solely on how a statement is worded.