Anyway this is a better test I found, but I can't seem to see the link anymore. Using my bash history for reference.
dd if=/dev/urandom of=tmpfile bs=1M count=10 && sync hdparm --fibmap tmpfile hdparm --read-sector [address between begin_LBA and end_LBA of previous command] # expecting random numbers here rm tmpfile && sync && sleep 120 hdparm --read-sector [address between begin_LBA and end_LBA of previous command] # expecting zeroesThe real key is the sector address is somewhere in between and NOT the start sector, as its possible the trim command will not set things to zeroes if files overlap. As it had happened to me 9 out of 10 tries using the test below I get random numbers still. The test above I get zeroes consistently.