Update

There is a way to test it answered at unix.stackexchange.com by frostschutz (this answer is his merit so thanks him), copied below:

"Create a test file: (not random on purpose)

# yes | dd iflag=fullblock bs=1M count=1 of=trim.test 

Get the address: (exact command may have to differ depending on filefrag version)

# filefrag -s -v trim.test

File size of trim.test is 1048576 (256 blocks, blocksize 4096)

 ext logical physical expected length flags

   0       0    34048             256 eof

trim.test: 1 extent found

Get the device:

# df trim.test

/dev/mapper/something  32896880 11722824  20838512   37% /

With this set up, you have a file trim.test filles with yes-pattern on /dev/mapper/something at address 34048 with length of 256 blocks of 4096 bytes.

Reading that from the device directly should produce the yes-pattern:

# dd bs=4096 skip=34048 count=256 if=/dev/mapper/something | hexdump -C

00000000  79 0a 79 0a 79 0a 79 0a  79 0a 79 0a 79 0a 79 0a  |y.y.y.y.y.y.y.y.|

*

00100000

If TRIM is enabled, this pattern should change when you delete the file. Note that caches need to be dropped also, otherwise dd will not re-read the data from disk.

# rm trim.test

# sync

# fstrim -v /mount/point/ # when not using 'discard' mount option

# echo 1 > /proc/sys/vm/drop_caches

# dd bs=4096 skip=34048 count=256 if=/dev/mapper/something | hexdump -C

On most SSD that would result in a zero pattern:

00000000  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|

*

00100000

If encryption is involved, you will see a random pattern instead:

00000000  1f c9 55 7d 07 15 00 d1  4a 1c 41 1a 43 84 15 c0  |..U}....J.A.C...|

00000010  24 35 37 fe 05 f7 43 93  1e f4 3c cc d8 83 44 ad  |$57...C...<...D.|

00000020  46 80 c2 26 13 06 dc 20  7e 22 e4 94 21 7c 8b 2c  |F..&... ~"..!|.,|

That's because physically trimmed, the crypto layer reads zeroes and decrypts those zeroes to "random" data.

If the yes-pattern persists, most likely no trimming has been done."

This is my automated script:

#!/bin/bash

#

# This script is provided "as is" without warranty of any kind, either expressed or implied, including, but not limited to, the implied warranties of merchantability, fitness for a particular purpose, or non-infringement.

#

# License GPL2

#

# by desgua 2014/04/29



function CLEAN {

cd "$pasta"

[ -f test-trim-by-desgua ] && rm test-trim-by-desgua && echo "Temp file removed"

echo "Goodbye"

exit 0

}



trap 'echo ; echo "Aborted." ; CLEAN; echo ; exit 0' INT HUP



if [[ "$(echo $USER)" != "root" ]]; then



read -n 1 -p 'Become root? [Y/n]' a

    if [[ $a == "Y" || $a == "y" || $a == "" ]]; then

        sudo $0 $1

        exit 0

    else

        echo "

        This script needs root privilege.

        "

        exit 1



    fi



fi





name=$(echo $0 | sed 's/.*///')

if [ $# -ne 1 ]; then



echo "

Usage: $name /folder/to/test/



"

exit 1

fi



pasta=$1



read -n 1 -p 'Use fstrim? [y/N]' a

if [[ $a == "Y" || $a == "y" ]]; then

    fs=1

fi



method=

while [[ "$method" != "1" && "$method" != "2" ]]; do

read -n 1 -s -p 'Choose a method:

[1] hdparm (will fail in LUKS on LVM)

[2] filefrag (warning: you may have to force quit - close the terminal - in some cases of success trim if you see an output that never ends) 

' method

done



function SDATEST {

disk=$(fdisk -l | grep /dev/sda)

if [ "$disk" == "" ]; then

echo "

fdisk did not found /dev/sda 

"

exit 1

fi

}



function TEST {

echo "Entrying /" ; echo

cd $pasta

echo "Creating the file test-trim-by-desgua at $pasta" ; echo

dd if=/dev/urandom of=test-trim-by-desgua count=10 bs=512k

echo "Syncing and sleeping 2 seconds." ; echo

sync

sleep 2



hdparm --fibmap test-trim-by-desgua

lbab=$(hdparm --fibmap test-trim-by-desgua | tail -n1 | awk '{ print $2 }')



echo "As you can see, the file was created and its LBA begins at $lbab" ; echo



echo "Syncing and sleeping 2 seconds." ; echo

sync

sleep 2



echo "Removing file test-trim-by-desgua" ; echo 

rm test-trim-by-desgua



trap 'echo ; echo ; echo "Aborted." ; echo ; exit 0' INT

echo "Syncing and sleeping 2 seconds." ; echo

sync

sleep 2



if [[ "$fs" == "1" ]]; then 

    echo "fstrim $pasta && sleep 2" ; echo

    fstrim $pasta

    sleep 2

fi



echo "This is readed from sector $lbab: "

hdparm --read-sector $lbab /dev/sda



pass=$(hdparm --read-sector $lbab /dev/sda | grep "0000 0000 0000 0000")



if [[ $pass == "" ]]; then

    echo "

Trim failed... 

You should see only 0000 0000 0000 0000 ...

"

else

    echo "Success!!!"

fi

exit 0



}



function LUKSTEST {

# Reference: https://unix.stackexchange.com/questions/85865/trim-with-lvm-and-dm-crypt#

echo 1 > /proc/sys/vm/drop_caches

cd $pasta

echo "Creating a "yes" file."

yes | dd iflag=fullblock bs=1M count=1 of=test-trim-by-desgua



#position=`filefrag -s -v test-trim-by-desgua | grep "eof" | awk '{ print $3 }'`

position=`filefrag -s -v test-trim-by-desgua | grep "eof" | sed 's| ||g ; s|.*255:|| ; s|...*||'`

[[ "$position" == "" ]] && echo "Could not find the position of the file. Are you on a LUKS on LVM?" && CLEAN;



device=`df test-trim-by-desgua | grep "dev/" | awk '{ print $1 }'`



yes=`dd bs=4096 skip=$position count=256 if=$device | hexdump -C`



echo "In the next line you should see a pattern like: 

00000000  79 0a 79 0a 79 0a 79 0a  79 0a 79 0a 79 0a 79 0a  |y.y.y.y.y.y.y.y.|

$yes

"



if [[ "`echo "$yes" | grep "y.y.y"`" == "" ]]; then 

    echo "The pattern could not be checked. Something went wrong. Exiting."

    CLEAN;

else

    echo "Pattern confirmed."

fi



echo "Removing the temp file." 

rm test-trim-by-desgua



echo "Syncing."

sync

sleep 1



if [[ "$fs" == "1" ]]; then 

    echo "fstrim -v $pasta && sleep 2" ; echo

    fstrim -v $pasta

    sleep 2

fi



# Drop cache

echo 1 > /proc/sys/vm/drop_caches



echo "In the next line you should **NOT** see a yes pattern like: 

00000000  79 0a 79 0a 79 0a 79 0a  79 0a 79 0a 79 0a 79 0a  |y.y.y.y.y.y.y.y.| 

If you see, then trim is not working:

`dd bs=4096 skip=$position count=256 if=$device | hexdump -C`"



yes=`dd bs=4096 skip=$position count=256 if=$device | hexdump -C`

if [[ "`echo "$yes" | grep "y.y.y"`" != "" ]]; then 

    echo "TRIM not working."

else

    echo "TRIM is working!"

fi

CLEAN;

}



if [[ "$method" == "1" ]]; then

    SDATEST;

    TEST;

elif [[ "$method" == "2" ]]; then

    LUKSTEST;

fi

exit 0