Use the Preflight panel. You can view the number of errors on the status line at the bottom of your screen. Double-click the error, or choose Window > Preflight
If Preflight is turned on, it will reveal different kinds of errors, including overset text. Click on the blue link to take you to the page where the error occurs, and it should highlight it. Also click Info at the bottom to get a message about the error.
Overset text means there is a text in the document that is not visible because your text frame is too small to hold all of the text that has been added. Frames with overset text will show a red Plus sign in the outport box on the lower right edge.
In addition to what Steve has told you, you can open the story with the overset text in Story Editor (Edit > Edit in Story Editor) and see what that overset text is. To correct the overset you would need to either enlarge the frame, add another frame to the text frame, or edit the text in some way to eliminate the excess.