There are two types of observations, subjective and objective. Telling the difference between subjective and objective observations is relatively simple once you are aware of what you are looking for.
The key differences between subjective and objective observations are exactly as the words imply, "subjective" meaning within the mind and "objective" which has actual material existence in the real world.
If something is subjective, it relates to a person's inner feelings, opinions, and thoughts. Something that is subjective cannot be proven or dis-proven, you cannot prove a person has a certain thought or feeling about something nor can you say with any certainty that they do not think or feel a certain way. For example if I were to mention that I had a headache without anything to go on other than my words, you could not know if it were true or not, it is subjective.
Something that is objective can be proven or dis-proven. If I were to say something were gooey and you were to touch and feel that yes it is indeed gooey, it would be an objective observation. Objective observations are material and can be felt, seen, tasted etc. If I were to say I had a headache and you could see that a large nail was sticking into my head, you could safely assume that my head were aching. Your observation would be objective based upon the material evidence, being the nail. A person may indicate that they are tired simply by stating that they are feeling tired. Without any physical evidence, yawning, drooping eyelids, slow movements... there is not any way to truly know if it is fact or not, making this subjective. If the same person were to begin yawning or better yet, to fall asleep, it will have become objective, because if they have fallen asleep it has been proven that they were tired.
Veterinarians are experts at objective observations, their patients cannot tell them where it hurts, therefore they must rely upon physical material evidence of medical problems. A medical doctor has the benefit of being able to take a patient's subjective observations into account in addition to their own objective observations before formulating a diagnosis. Another profession that requires relying heavily upon objective observation is law enforcement. It is never wise to form an opinion with only subjective observations to go on, an officer of the law must rely upon objective observations, which may be proven.
Remember when deciding if an observation is either subjective or objective, take into account whether you see, taste or feel evidence. If so, you have an objective observation. If the observation is based only upon opinion thought or feeling, it is subjective and cannot be proven one way or the other.