OK......well and good for someone knowing the code and all. But, in
regular do-it-yourself language....how can I tell if a breaker box is
OK with a tandem breaker?

If that breakers model/part number is not listed on the label in the
panel cabinet and that panel's model/part number is not listed on a list
that is packed with the breaker then you cannot use that breaker in that
panel.
--
Tom H