How can one decide between 'not set' and 'not existing?

[meino.cramer@...]

Hi,

Suppose one do

a = { key = nil }
print( a.key )

=> nil

print( a.carkey )

=> nil

It looks like setting a key in a hash with value nil is the
same as if the key does not exist at all. 

How can I decide between both in a program?
Is there a 'exists' function as in Perl?

Best regards and thank you very much in advance for any help!
mcc