I'm assuming you're working on a dentoform and not real human. Close one eye and look straight down the prep with the other eye; you will easily see the undercut.
If you're having problems after closing one eye, some people will mark around their prepared margin on the typodont. This may help some in training your eye initially... If you cant see the entire margin- you have an undercut, obviously.
in addition to the above listed ideas (which are great), you can take a straignt instrument (e.g. a probe) and run it flush along the wall of your prep to find gaps, though this may be tough along your interproximal surfaces.
Take a pencil, mark the interior line angles of your prep. If you look straight down your prep and can't see the pencil mark in certain areas, then its undercut there.