Open your passport to the second page. This should be a glossy page where you see your picture. The next page should have your signature on it.
Turn the passport vertically so it is easier to read.
Locate the second to last piece of information next to your picture. The title should be "Date of expiration/Date d'expiration/Fecha de caducidad." Under that phrase should be a date in the format of day/month/year. This is the expiration date of your passport. See photo for reference.