I am the kind of person who will tell you if your fly is down or if you have something in your teeth. I don't do it to be critical or point out shortfalls. Rather, I think it's the helpful thing to do, and I know I would want someone to tell me. The other day, I went into a meeting and afterwards noticed I had spinach in my teeth. I can't believe no one told me! How can I communicate to other people that I really do want them to tell me if something is amiss in my own appearance? And how can I share this kind of information with other people in a way that doesn't offend them?
Given the prevalance of new technologies, maybe it's time someone invented a "spinach detector" or "check your fly" app. Until then, we have to rely on our friends, or our mirrors, to alert us if anything's off about our appearance.
You're right to point out that there's a delicate balance in how this information gets shared. Perhaps as you make known that you want this information shared with you and are increasingly on the receiving ends of these comments, you'll get some ideas of how to be on the giving end, too.
In my experience, some phrases that work well include, "You might want to check the mirror," and, "Would it be helpful if I pointed something out to you?"
If you have a streak of sunscreen across your forehead, it's not that embarassing to have someone point out that you need to rub it in. If someone points out the spinach before an interview, it's awkward, but better than the potential employer pointing it out mid-interview, or, worse, no one pointing it out at all. In the case of an unzipped fly, pulling someone aside and whispering, "Check your fly," is the kindest and most discrete way of handling the situation.
Whenever you share one of these little unfortunate facts of life with someone, you could follow up with, "I hope it's OK that I mentioned it because I know I'd want you to tell me," or, "These sorts of things happen to me all the time, and I'm always relieved when someone helps me out."
You could make an extra point to tell those closest to you not to let you leave the house with your slip tucked into your skirt or not to ignore the ketchup stain on your shirt out of politeness. You can be as direct as you want about how direct you want people to be with you. But you need to be as gentle as possible with others, unless you have a prearranged blutness agreement.
While it's nice to be able to rely on others for a little nudge to improve your appearance, you can also take more responsibility yourself. App or not, before a meeting, set a reminder on your phone to check your teeth. Check your fly when you leave the bathroom. Create a checklist of likely issues you face when leaving the house in the morning so you'll do one last onceover before heading off to work.
Finally, as much as these potential embarrassments are at the forefront of your mind, remember that not everyone is as fixated on your appearance as you are. It's actually possible that no one noticed the spinach in your teeth. We're all much more focused on ourselves than we may realize. We're certainly more focused on ourselves than anyone else is focused on us.
You're more equipped to notice when something is out of place and also more likely to notice something that is of no consequence to anyone else. Cut yourself, and everyone else, some slack, and go eat a salad with abandon.