<p>I'm hoping there's something in the same conceptual space as the old VB6 <code>IsNumeric()</code> function?</p>

Try the
isNan function
:
The isNaN() function determines whether a value is an illegal number (Not-a-Number).
This function returns true if the value equates to NaN. Otherwise it returns false.
This function is different from the Number specific
Number.isNaN()
method.
The global isNaN() function, converts the tested value to a Number, then tests it.
Number.isNan() does not convert the values to a Number, and will not return true for any value that is not of the type Number...