First, look for a group of words without a subject. In English a sentence is usually written in the order of Subject Verb Object. If the subject noun is missing then you will know you have a fragment.
Then look for a group of words without a predicate, especially a group that contains a verbal or a verbal phrase. Another word for a predicate is a verb phrase and for a group of words to be a sentence it must have a verb. If it doesn't then it is a fragment.
Finally be sure you haven't punctuated a subordinate clause as if it were a complete sentence. Some proeple think that by punctuating a sentence it makes it correct, without conforming to the conventions or syntax of the language.
Many times, you can correct a sentence fragment by attaching it to a clause. Other times, you may need to add words to make the sentence complete. EG. (Fragment) Danielle is on the basketball team. Played for fun as a child. (Complete Sentence) Danielle is on the basketball team. She played for fun as a child.