>First, how can I check  that the variable  InitialRange is a reference
>to cells  in the spreadsheet  (e.g. "=Truncate(B6:B12)"), and not a
>number or a string  (e.g. "Truncate(6)")?

In code, you can use functions  such as:
TypeName()
VarType()
IsObject()

During debugging, you can see the Type of everything in the locals
window if you set a breakpoint.

>Is vba  in any way a typed
>language?

Yes, but it has a default type called Variant that can morph into
whatever type is expected by the code. Unless declared otherwise,
variables are Variants.

>Once that is done, how do I access  the values  in the range?

Here is an example  of a function  that returns Boolean values (e.g.
to the spreadsheet or another function or subroutine):

Function Test2(x)
a = x 'this is a trick to convert a range  object to a variant
array
NRows = UBound(a, 1)
NCols = UBound(a, 2)
ReDim b(1 To NRows, 1 To NCols)
For r = 1 To NRows
For c = 1 To NCols
b(r, c) = a(r, c) < 100
Next
Next
Test2 = b
End Function

>Finally, I want to return a new range that is a subset of
InitialRange.

The above example returns values (e.g. for use in the spreadsheet).
If you want to return a Range Collection instead of values (e.g. for
use in another code block), look at the Range Collection, Offset and
Union methods in Excel help.