Silver maples are so called because the undersides of their leaves appear silvery-white. In June, their leaves are fresh and fully-developed, and the trees look at their best, especially if there's a breeze causing them to flutter and show silver flashes.
Silver maple leaves otherwise resemble other common maples including Sycamore and Norway maple, so it is their undersides that distinguish them from these other common species. Another time to look out for them is late February to early March when they come into flower with red inflorescences that can be quite distinctive, but are not as eye-catching as a flowering cherry, so many people may not notice them.
They can become very large trees, so most street trees have been pollarded to maintain a manageeble shape and size.