Japanese stamps issued both before and after 1947 have distinguishing characteristics. Most post-1947 stamps include the word “Nippon” written on it. Japanese stamps pre-1947 usually bear the imperial chrysanthemum crest.
Look for the word “Nippon” written anywhere on the stamp (looks like a capital "E" with an extra, vertical line running down the right side.) Most stamps made in post-1947 Japan bear the name of the country in roman script.
If you cannot find the word “Nippon” on the stamp, check for any logographic (“picture”) writing. If the stamp does bear any logographic writing, it is likely Chinese style script (called “kanji”), which is used in China and Japan.
Look for one of two kanji characters. One Is a box, long sides up, with a horizontal line cutting it in half (likely made in Japan after 1947). The other is a box, long sides horizontal, with a vertical line running through it (likely made in China).
Search the stamp for a chrysanthemum. (Search online for a picture if you don't know what this flower looks like.) The Japanese imperial chrysanthemum is a stylized circle with 16 petals radiating out from a smaller circle in the center. If the stamp bears this crest, it was likely issued in Japan prior to 1947.