You can usually see when a nail has been picked/nibbled off and eventually you will know your clients well enough to know when there is a genuine problem. I give everyone the same information - ALL repairs are chargeable. It's then entirely at my digression whether I give it for free or not. It's VERY rare that I repair for free. I do feel quite lucky that the majority of my clients really do take care for their nails and are usually very honest about how any damage has occurred. When they say "That one just came off", I know them well enough now to say "Did it perhaps have a little help?" and they usually say yes!