Learn the seven letter names that repeat across the piano, using the black-key pattern as your guide.
Give the keys their names
Find C
Look immediately to the left of a group of two black keys. That white key is C.
Walk through the letters
Moving right from C, play C D E F G A B, then C again. The letter sequence starts over after G.
Find the same note elsewhere
Choose one letter and find it in several octaves. Use the nearby black-key groups instead of counting every key.
The same letter name appears in different registers. Those notes share a name but sound higher or lower. Add sharps and flats after you can confidently locate the natural letter names.