Overset text in InDesign occurs when a text box is too small for the type contained in it, resulting in missing words, paragraphs, or even entire pages in a document. A clear sign of overset text is when the text frame shows a red plus sign in the bottom right. To find and fix overset text in InDesign, there are several methods:

1. Use the Preflight panel: This panel displays a red circle in the lower-left corner of the document window whenever there is overset text. Clicking on the page number will bring up the overset text on the screen.

2. Use the Story Editor: Select a text frame and choose Edit > Edit in Story Editor. The entire overset text will be marked by a red line on the vertical depth ruler.

3. Use threaded text frames: By threading text boxes, text from a first box flows into a second one, allowing for more text to fit in a document.

Once overset text is identified, it can be fixed by editing, highlighting, decreasing the font, copying, cutting, or deleting it if not needed.