Was riding up a pass in CO one summer on a week long tour. Passed a rather robust gal who was really struggling to get up the hill. I noticed she still had another lower gear left to go and was not in it. I watched for a bit to see if she would shift but she did not. I then, as politely as I could, pointed out to her she had another gear left. She looked down and quickly shifted. She thanked me and said she thought she was in her lowest gear and thanked me again. We chatted for a minute and then I moved on.
Sometimes it goes well.