asserting types

[Mick]

What is the cleanest way to assert a class type?  for example if one has a
function that uses a particular class object then what is the nicest way to
assert within the function that the class is of the correct type?  I assume
this is simple, so I guess more the question is this the wrong way to think?
should I be trying to organise things in a different way so as to generate
"compile" time errors?

regards
Mick