The common answer on the Internet is to read the spec (they never mention it) or know the chipset (hard to know before buying) before buying. Where can I find a list of SD card readers which supports this feature and allows the use of
If the device can't work with blkdiscard, which is true for all my USB based card-reader (20 of them), then you get the following:
# blkdiscard /dev/sdb blkdiscard: /dev/sdb: BLKDISCARD ioctl failed: Operation not supported
blkdiscard works with TRIM. As far as I can see to support TRIM via USB bridge the USB bridge chip MUST support SCSI / ATA Translation(SAT)'s ATA PASS THROUGH command in order to pass the TRIM command to the disk, so this is effectively asking which USB based card reader supports ATA PASS THROUGH.