If the top end of the 6 is farther to the right than the bottom end, it is a large date. On a small date the 2 ends are close to vertical. After you have seen enough of them, you can notice the height easier.
Large Date, sorry. Here's a good resource on small date 1960 P&D Lincolns: http://www.lincolncentresource.com/smalldates/1960smalldate.html