Read over a piece of writing that you would like to check and find any sentences which contain two independent clauses in one sentence without proper separation. An independent clause is one that contains a subject, a predicate, and a complete idea. In other words, the independent clause has someone or something which effects an action and the action itself. It also communicates a full idea. For example, "The cat hunted," is an independent clause because it contains the subject ("the cat"), a predicate ("hunted"), and an understandable idea. A sentence with two independent clauses that do not have proper punctuation -- a comma and a coordinating conjunction or a semicolon -- may look like: "Bananas grow on the tree I harvest them." This is a run-on sentence.
Check each sentence with two independent clauses to see whether it qualifies as a run-on sentence. Run-on sentences do not properly join two separate ideas, but instead mix them together without punctuation or conjunction. One way to test for run-on sentences when you are unsure about the identity of a sentence is to turn your sentence into a yes or no question. For example, "Do bananas grow on the tree I harvest them?" In this instance, "Bananas grow on the tree," and "I harvest them," each represent two independent clauses. Speaking the sentence aloud also helps you to hear when a sentence sounds grammatically correct.
Transform a sentence in a tag question as another way of checking it. A tag question is a question with a clause that ends in a questioning phrase such as, "Bananas grow on the tree I harvest them, don't they?" Another example is, "The cat hunted he is very quiet, isn't he?" In this way, it becomes apparent that these sentences do not work because they include two independent clauses without correctly conjoining those clauses, and are therefore run-on sentences.