Outline the plot you have created in a chronological manner. Add a few notes of detail to each plot point to remind yourself of the action of that part of the story. Look to see if parts of the story jump from one sequence to another without a connecting piece of information.
Follow a simple point-by-point story check. Determine where the characters begin, then take them to point B, point C and so on until they reach the conclusion of the story. If there is a spot in the story that action seems to happen for no reason without a connection to the last point, you have plot hole that needs filling. Create a reason or piece of action to explain that situation.
Ask questions about the motivation of your characters. If they need to have a conversation explaining part of the story, ask why they need to have this conversation. If a character needs to do something that doesn't seem like their normal action, ask yourself why and how they end up in that situation. If something needs to not happen, say two characters meeting, ask how or why that might be delayed.
Answer the questions you posed as to why action or plot points need to happen with believable reasons or actions that add to the story. Creating tension and reasons for characters to either do or not to do something is an excellent way to fill a plot hole and add excitement to your story line. It gives the audience an answer as to why something is not happening and at the same time creates anticipation. Create an action such as an accident or weather situation to delay a person getting somewhere. Concoct a back story as to why a person might have a behavior by a childhood trauma or other believable reason that will add tension without confusing the existing plot.
Go back to the outline and write in the pieces where you have created an action or sequence to answer the open areas of the plot. Read through the outline again and make sure it flows smoothly. Check to see if now the action flows from point to point without gaps. Also check to be sure you have not created any more new holes by setting up an action that now needs new answers as to why it occurs.