[Triggers] Check if OLD or NEW records are NULL

Dear All,

Can I check if OLD record is NULL in trigger function.
I'm using tg_op and check if it's UPDATE, etc. but I need check if OLD
record exists.
Is it possible?

f.i.

--------------------------------------------
IF OLD IS NOT NULL THEN

IF OLD.identity IS NULL THEN
tg_op_final:=''INSERT'';
ELSE
tg_op_final:=''UPDATE'';
END IF;

ELSE
tg_op_final:=''FILL_BY_DATALOADER'';
END IF;
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Thanks!
--
Best Regards
Maciek