Linux will detects SSD automatically. Since kernel version 2.6.29, you can check /dev/sda with following command
1 # cat /sys/block/sda/queue/rotational The return number 0 shows you /dev/sda is a SSD, and 1 shows it is a HDD. Note that this command may not work when your disk is created by hardware RAID.
Another way is to use lsblk command, a part of the util-linux package.
1 2 3 4 # lsblk -d -o name,rota NAME ROTA sda 0 sdb 1 ROTA means rotational device, 1 for true, 0 for false.