You can create a line break within a cell using Alt+Enter. The character that breaks the line is ascii character 10. To find cells with a line break, you can use Edit>Find. To get the line break character in the “Find What” box, type Alt+0010 (that is, hold down the Alt key and type 0010 from the numeric keypad) with your cursor in that box.
You won’t see anything in the Find What box when you do this, however, so if it doesn’t find anything it’s likely that you typed in the wrong number.
Don’t foget to use the numeric keypad, it won’t work with the numbers at the top of the keyboard.