You may put a device, such as a serial mouse (use 1200 baud), on a port and then use minicom or picocom to communicate with that port. Then by clicking on the mouse, or otherwise sending characters with the device, see if they get displayed. It not you may have told picocom the wrong port (such as ttyS0 instead of ttyS1) so try again.
Missing connectors
If the software shows that you have more serial ports than you have connectors for (including an internal modem which counts as a serial port) then you may have a serial port that has no connector. Some motherboards come with a serial port with no cable or external serial DB connector. Someone may build a PC from this and decide not to use this serial port. There may be a "serial" connector and label on the motherboard but no ribbon cable connects to its pins. To use this port you must get a ribbon cable and connector. I've seen different wiring arrangements for such ribbon cables so beware.
