var special_ent_created = 0;
..
void _myaction(){
   special_ent_created = 1;
}
void _creator(){
   ent_create ("any.mdl", vector (0,0,0), _myaction);
}
..
if (special_ent_created) { Entity "special_ent_created" was created }

ENTITY* myEnt = null;
..
//somewhere in your code
myEnt = ent_create ("any.mdl", vector (0,0,0), <some action of your own>);
..
//another place in your code
if (myEnt != null) {
//your entity was created.
 }

if(ent_create("any.mdl", vector(0, 0, 0), foobar))
{
    // Your code here. You can use the you pointer to access the created entity
}

if((bar = ent_create("any.mdl", vector(0, 0, 0), foobar)))
{
   // Your code here.
   // Note: You don't need the extra pair of parenthesizes around the if(),
   // but any sophisticated compiler will warn about the assignment otherwise 
}