Check in Control Panels for the Startup Items control panel. If there is actually an OS X OS installed, it will show the System folder there in the list. If not, then it's not installed.
BTW, if it's anything less than 10.2, don't even bother booting into it. You won't gain anything from versions before 10.2. And before you do this, check to make sure you have enough memory to run the OS X portion. At least 128 MB is required for the OS to run, 256 MB for any actual USE of the operating system.
Also make sure that you have the necessary firmware updates installed on that Mac, otherwise you'll have problems.
• Apple iMac G5 17" (2 GHz G5) - Mac OS X 10.5.8/Ubuntu 10.04
• Asus Eee PC 901 (1.6 GHz Atom N270) - Fedora 13
• Apple Macintosh Quadra 650 (33 MHz MC68040) - Mac OS 8.1
• "JHVH-1" (2 GHz AMD Athlon XP 2400+) - Slackware 13.1