To find options, go to the File menu, then click options, then click advanced and click on comments and indications.
You can use conditional formatting to highlight cells that have comments.
Or you can scan through the whole sheet for those tiny little red flags.
Or you can use ctrl+F to search for cells that have comments.
Or you can just make them show by default by ticking a simple box located on the file tab Excel->Options->Advanced->Display and ticking the third radiobutton.