The one time we can notice the variation in lunar angular diameter is during TOTAL/ANNULAR SOLAR ECLIPSES: the Moon's angular size must be smaller during an annular eclipse than during a total eclipse. During total/annular solar eclipses, the Sun provides a standard of comparison.
          There are some further complications to the changing angular diameter of the Moon that are well discussed in at the image credit site.
Credit: John Walker's Inconstant Moon page. John Walker has declared this page and its images public domain.