Step-by-step explanation:
One of the easiest ways to tell is try reading the sentence out loud. You shouldn't be running out of breath reading a sentence. Another way is when there is a new point being made, or a slight change in the topic or subject, and there is a period, or comma missing, that would be a run on sentence.
Example below
Another way is when there is a new point being made or a slight change in the topic or subject and there is a period or comma missing that would be a run on sentence.