determining an objects type?

HI, i'm sure I read something, even produced an example for myself on this. but I can not seem to find it at present.

the problem is this, determining if an object belongs to a particular type

for example (this is not leagal code)

strict

type foo
  field x
end type

local x : foo = new foo

if x = foo() then print "x is a foo"
end

can anyone enlighten me on how to do this correctly.

cheers.