Please use the [code ] tag in future posts.
Define a skill that tells you what type of entity it is:
#define _type skill100
#define type_player 1
#define type_enemy 2
#define type_bullet 3
Now, in your player, enemy and bullet action, you set _type to the appropriate value:
function bullet() {
my._type = type_bullet;
...
}
Finally, you are able to identify the type of object in your collision_event:
if(your._type == type_bullet) { ... }