Like I said, I'm not too familiar with ACPI, so I can't be of much help with that. To see if your computer is using APM, you could use
that will give you APM status information if APM is in use. How you enable it depends on your distribution. You need at least apm kernel module. You can load it with:
There might be an initialization script in your distro that does it for you in Slackware it's in /etc/rc.d/rc.modules, you will need to open the file in text-editor and remove the # mark from that line. Your distribution might be different. You could try looking for /etc/rc.d/init.d/rc.apm or rc.apmd, or /etc/init.d/rc.apm or rc.apmd. In distributions using System V style init, you enable it using chkconfig. man chkconfig will probably help you with that.
In addition to kernel module, you will need apm-daemon.
If you system has rc.apm(d) somewhere, I think it will load the daemon automatically. Slackware will start apm daemon automatically if the apm kernel module is loaded, so you don't need to anything about that.
Furthermore, you will probably want to use apm-proxy file (to unmount those filesystems and such). The standard location for that is /etc/apm/apmd_proxy. In your distro it may be somewhere else or it might not exist at all (in which case I can show how it looks like, let me know if you need it).
You will also need to disble ACPI if you plan on using APM, the two should not be used simultaneously. Maybe it's /etc/rc.d/init.d/rc.acpi (or /etc/init.d/rc.acpi), or something like that, but I'm only guessing here. If it is, you can disable it using chkconfig. In Slackware you just:
chmod a-x /etc/rc.d/rc.acpi