How to check if a symbol has been defined?

[flamasuta]

Is it possible to check if a symbol has already been defined?

For example, in pseudo-code:
if (symbol is defined) then
   do that thing
else
   do another thing
endif

Or, at least, is it possible to re-define a symbol (only if it is 
currently undefined)?

I've tried to use @If and @Case but...

Thank you.