That can be difficult.

The London planetree (Platanus × hispanica, syn. Platanus × acerifloia) is the tough, hybrid street tree that is known for its pollution tolerance, peeling bark and large, shady canopy. It is the most common street tree in New York City numbering over 100,000 plants.

American sycamore, also called American planetree, (Platanus occidentalis) is a northeast native plant that is one of the species hybridized to create London planetree.

If you are looking at a tree in New York City, it is more likely to be a London planetree but there are also over 1,500 American sycamore trees among the street trees in the city.

Both trees have fuzzy balls of fruit and mottled peeling bark.  The leaves and the way that the fruits are displayed in the fall are the best identifiers for these two trees. The leaves of London planetree look somewhat more like a maple leaf with three to five pointy lobes whereas American sycamore typically has a broader, less deeply incised leaf that is most often three lobed and slightly fuzzy along the veins on the underside when mature.

The fuzzy seed balls are syncarps made up of numerous single-seeded fruits called achenes. London planetree typically has multiple syncarps per stem in fall and the American sycamore usually has just one per stem. These features stay on the tree even after leaves have fallen.

The bark of each tree peels to make beautiful, exfoliating patterns on the trunk as the tree matures. In general, American sycamore is described as having more exfoliation on the upper parts of the trunk where creamy white inner bark may be exposed and London planetree has more uniformly peeling bark in cream, olive and light brown layers along the length of the trunk.  Bark and the pattern in which it sheds can vary so much, however, from tree to tree that this is a less clear way to distinguish between trees without also examining leaves and fruit. 

Courtesy of NYBG Plant Information Service