If you are on a UNIX system or on windows with cygwin you can run:
grep -r -e "@family\s*$" ./
from the directory with your files. If not most text editors have a regex find in files option.
This doesn't guarantee a hit in case the problem is that your family tag is malformed as opposed to just missing, but will probably work. If it is the latter problem then you should start by what @BenBolker suggested.