First, be sure manufacturer’s name appears on the iron plate, soundboard, or somewhere else besides the key fallboard.
Second, look for the serial number. Older pianos have 4 or 5 digits, newer ones 6, 7 or 8 digits sometimes starting with a letter. Piano manufactures place serial numbers in a variety of places such as plate, plate ribs, soundboard, bridges, capo d`astro bar, inside rim, back posts etc.
If you cannot find it, ask your piano tuner or local dealer for help. Sometimes the action will have a date of manufacture written on it.
Third, give the piano name and the serial number to your piano turner to identify the manufacturing year, if it is listed in the reference books.