Identifying feelings means being able to know when you are feeling sad or down and is a pre-requisite before being able to do something about it (regulating your feelings and emotions)
Your little sister is rolling around on the floor giggling
To identify feelings and emotions pay attention to the person’s facial expression; look at the person’s eyes, mouth and eyebrows to determine how they are feeling (smiling when happy etc). Body language can also help you determine how someone is feeling (arms crossed when angry etc). Lastly you can listen to the person, their voice tone will also give you clues.