Structuring Test Data
Since you're on NUnit, why don't you just use its built-in TestCase attribute? Your test would then look like this:

[&lt;TestCase("123", true)&gt;]
[&lt;TestCase("Foo", false)&gt;]
[&lt;TestCase("123Foo123", false)&gt;]
let ``IsNumericTest``(candidate : string, expected : bool) =
    let actual = isNumeric candidate
    actual |&gt; should equal expected


xUnit.net (a better unit testing framework for .NET) has a similar feature.