How to test if range selected fits criteria

I have some code that runs on 'with selection', like this:

With Selection
'do this
end with

I am looking for a way to check that the selection is a range that is "A?:M?"
e.g. 'A1:M10', or 'A5:M45' etc

so that my code becomes something like this:

with selection
 if selection is range("A?:M?") then
  do this
 else
  exit sub
 end if
end with

Any assistance appreciated.