Per Daniel's suggestion, resharper supports a good deal of annotations to assist with inspection.
Specifically, via the documentation what we're looking for here is something like:
[ContractAnnotation("d:null => false")]
static public bool IsValid(double? d)
{
return d != null && IsValid(d.Value);
}
Which does the trick perfectly, and the static check works beautifully.
Love that resharper!