ls -al /dev/ttyS?

# returns
crw--w---- 1 root tty     238, 0 May 29 15:23 /dev/ttyS0
crw-rw---- 1 root dialout 238, 1 Jan 29  2018 /dev/ttyS1
crw-rw---- 1 root dialout 238, 2 May 29 15:28 /dev/ttyS2