I don't think that a plugin would be reliable to determine the keys that
are used by a general arbitrary program. Imagine that your resx contains a
key named "string123" and your code does:

int i=123;
key ="string"+i;
x = GetGlobalResourceObject("WebResource",key);

  This uses your "string123", but how would an automated tool conclude this
from the source code? What if the "i" variable above is a computed value
that depends on inputs provided at runtime?