There are several different packages to do this. I tried upsd (directions below), but it doesn't work with my "dumb" UPS (APC 500). upsmon actually has some documentation, but it sounds like it won't either, so I won't even try. The latter mentions pwrfaild, but I can't find anything about it. Somewhere on the web I found reference to bkpupsd, so now I'm trying it (many gory details below).

upsd
• Plug a serial cable from the UPS port to a serial port on your FreeBSD box. Default is the lowest numbered serial port (probably labelled "Port 1" on the back of the box).
• cd /usr/ports/sysutils/upsd
• make
• make install
• Goto Autostarting and set up an autostart script for upsd. This script need not save the pid in /var/run, the program does this automatically.
• If necessary, edit /usr/local/etc/upsd.conf. The default serial port is /dev/cuaa0 (port 1, the lowest numbered).
• Now you'll get messages on the console that say "Cannot put UPS in smart mode". I guess this means I should have spent more money on my UPS. Mine is an APC BackUPS 500. Oh well...
• make clean
• make deinstall

bkpupsd (Pretty much the same install as upsd)
• Plug in control port.
• cd /usr/ports/sysutils/bkpupsd
• make
• make install
• Goto Autostarting and set up an autostart script for upsd. This script need not save the pid in /var/run, the program does this automatically.
• If desired, edit /usr/local/libexec/bkpups.*. These scripts are executed by the program when events happen.
• After you've done the install, you can use man 8 bkpupsd for more info.

I had to make a special cable to adapt my APC Back-UPS 500 work with this program. A DB-9 male to female (male is UPS side), with all pins just passing through, except:
pin 4 is left open on both sides, and pins 6 and 7 are shorted. The reason is that the UPS is asserting DTR (pin 4) and DSR. DTR is supposed to be driven by the computer end, thus it is being held low by the computer. This in turn, is holding DSR down as well, thus it is left open.

I also had to modify the program -- immediately before the line ioctl(fd, TIOCMBIS, &dtr_bit);, I had to add dtr_bit = -1; this is the first one inside the while(1) loop. Also, the line else if(lstatus & TIOCM_RNG) { gets the TIOCM_RNG replaced with 0x100.
Diff listing:

90a91
> dtr_bit = -1;
112c113
>          else if(lstatus & 0x100 /*TIOCM_RNG*/) {
---
<          else if(lstatus & TIOCM_RNG) {

• The "test" switch causes the same action (as far as the serial port is concerned) as does pulling the power plug.
• Depressing the "test" switch causes the serial status to change, and remain in that state as long as the button is held depressed. When the button is released, the status will return to normal.
• Pulling the power or depressing the test switch with no load on the UPS (nothing plugged in/turned on), the alarm tone will be continuous. With a load, the alarm will beep for about 1 second, about every 4 seconds.
• The UPS asserts DSR and DTR. The UPS also asserts RD. I know, this is bogus (DTR is DCE, DSR and RD are DCE), but that's the behaviour I observed from mine.
• DSR tracks DTR. If your computer asserts DTR (as mine does), this makes it so that no change is observable on DSR either.
• Upon "test" button or powerfail, RD, DSR, and DTR are all forced low. This means that by monitoring DSR you can test for powerfail/test-button.
• If your system were to monitor DTR, instead of driving it, you could monitor DTR to test for powerfail.
• You should also be able to test for powerfail by checking the recieved data for BREAK (eg. framing error).
• Just for the record, my APC 500 held up my computer (4 SCSI HDs spun down, full of RAM, Pentium II 266, internal CDROM idle, nothing else special) for 27 minutes. After that time, the alarm beep became continuous, and after another about 90 seconds, it shut off.
• Due to the amount of time it takes to do a battery charge/discharge cycle, I only did one test for low battery indication. However, the test was conclusive -- I did not observe any status change, meaning it is not possible to detect this status.