Since it is a kind of seasonal menu, cafes usually let people know if they have it. The small flag hanging on the signboard of a café in the upper left hand side photo is a typical traditional sign that advertises the cafe has shaved ice.
The red symbol on the flag is a Chinese character “Ice”. Chinese characters are pictographic so they can show an idea in a compact form. With this flag people can easily see if the café is serving Kakigoori(shaved ice) without entering the café.
However, nowadays some cafes serve Kakigoori (shaved ice) throughout a year. What's more, there are even Kakigoori cafes which serves shaved ice mainly, and of course they have the same flags at the entrances. They serve more fancy types Kakigoori as you can see in the lower photos.
If you want to try Kakigoori (shaved ice) in Japan, look for a cafe with the frag of "ice"!