fstrim for LUKS2 encyrypted LVM

[Marco Möller]

To my understanding, it makes no sense to perform a TRIM on storage 
which is a LUKS2 encyrypted LVM. The storage device should anyway think 
that each bit is in use after it was filled with random data when 
creating the space. Not only that I cannot imagine how the storage 
device should Know what is happening in the encrypted space, wouldn't it 
be a security issue if the OS would inform the storage device about 
unused space and its location and could actually perform some kind of a 
TRIM?

Am I wrong?
If I am right, then, and assuming that TRIM is done by a command called 
fstrim, is there a simple command by which I could search through all 
cron entries if fstrim would somewhere be defined to become executed?