Determine if function is defined with C API?

[Joey Mukherjee <joey@...>]

Is there a way from the C API to determine if a Lua function is defined?

If a Lua function is defined, I wish to call it, otherwise I want to  
default to some other behavior, but it is not really an error.

How would I do something like this?  From lua_pcall, I get a return  
value of 2 but is this defined somewhere?  Should I just check for  
that value or is there a preferred constant?

Thanks,
Joey