Absolutely. No one symptom can diagnose or rule out B12 deficiency. And fingernails are a good thing to pay attention to, as many correctible problems can be pointed to by their condition.
Here's a couple of helpful sites:
http://www.mayoclinic.com/health/nai...=false&ISAGG=0
http://www.aafp.org/afp/20040315/1417.html
rose