There's a little program (statserial) which reads serial port pins state.

I use it on Linux Debian and CentOS, but I can't find in /usr/ports. I don't know if it's only for Linux or it can be used also on FreeBSD.

If someone wants to check, I'll appreciate a lot.

Here are some references:
Summary: A tool which displays the status of serial port modem lines.
Name: statserial
Version: 1.1
License: BSD

The statserial utility displays a table of the signals on a standard
9-pin or 25-pin serial port and indicates the status of the
handshaking lines.  Statserial is useful for debugging serial port
and/or modem problems.
I gave it a try. The only change I did was to remove LDFLAGS out of the Makefile. This was a shot in the dark, but may be successful. Please see below the relevant section of the modified Makefile.
statserial:   statserial.o
   $(LD) -o statserial statserial.o -lcurses
make statserial generated some binary.
Running statserial /dev/cuau0 as root generated some output as below.
Device: /dev/cuau0

Signal  Pin  Pin  Direction  Status  Full
Name  (25) (9)  (computer)  Name
-----  ---  ---  ---------  ------  -----
FG  1  -  -  -  Frame Ground
TxD  2  3  out  -  Transmit Data
RxD  3  2  in  -  Receive  Data
RTS  4  7  out  1  Request To Send
CTS  5  8  in  0  Clear To Send
DSR  6  6  in  0  Data Set Ready
GND  7  5  -  -  Signal Ground
DCD  8  1  in  0  Data Carrier Detect
DTR  20  4  out  1  Data Terminal Ready
RI  22  9  in  0  Ring Indicator
which looks promising.
I get these errors: