entity exists?

Can you be more specific here ? What does it mean 'the entity fails' ???

Usually, an entity has a number associated. If you check that number you should be right:

if my_entity > 0 then 
;entity exists
else
;entity does not exist
endif

Note:
If you have a type element which has a field that you use as an entity pointer, and you want to delete that element type, then you should free the entity first, and then delete the type:

FreeEntity mytype\entity ;first release the entity
delete mytype ;then delete the type element