Does a field exist?

How do you find out if a dynamic field was initialized in script?

Ex.

%obj.field1 = 0;  
if ( exists(%obj.field1) )
{
     echo("Field exists."); 
}

if ( exists(%obj.fake) )
{
     echo("Field does not exist.");
}

I know dynamic fields get initialized to zero... but what if zero is a valid value in the context? Testing for zero won't work for me.