How do I know which /dev/ttyS* is my serial port?

I think it's this command:

dmesg | grep tty

Running that on my own Linux box (which only has 1 Serial port) produces a single ttyS0 output line. Try it on your own, you will see what I mean.

See which UARTs where detected in /proc/tty/driver/serial. A line with uart:unknown means: nothing detected (and likely not existent).

# cat /proc/tty/driver/serial 
serinfo:1.0 driver revision:
0: uart:16550A port:000003F8 irq:4 tx:0 rx:0
1: uart:16550A port:000002F8 irq:3 tx:111780 rx:1321 RTS|DTR|DSR
2: uart:unknown port:000003E8 irq:4
3: uart:unknown port:000002E8 irq:3

If something is connected and driving the lines CTS, DSR or CD (these are input lines) you can even be pretty sure that there actually is something... Same is true for the rx-byte-count.

If you need to do this programmatically reading the output from dmesg can be troublesome, instead the folder /dev/serial/by-id has sym links that are named after identifiable data of your device and point to the specific /dev/tty* they are connected to.

I'm not sure if this is some special udev rule that is distribution specific, but it works well in Ubuntu, let me know if it works.