New Followers DM but don't hit "follow"

I've been getting greeting type DMs from new followers, but NOT actual *follows*. How can I let them know they forgot to hit "follow". They say things like, "I look forward to your tweets" so I know they intend to follow. I can't figure out how to reach them by DM, because they're not yet on my DM list, and typing D = name doesn't work either. Please help.

Thanks so much!
NELLYWAITS