Most compatible models (except the 20GB version) have a flip-up flap on the front that hides memory card slots.
Compatible models have 4 USB ports on the front. If your console only has 2 USB ports, it is not officially backward compatible with PS2 discs.
These tutorials provide detailed walkthroughs for setting up PS2 emulation on various PS3 models using custom software: How to Play PS2 ISOs on a Jailbroken PS3 with Rebug CFW YouTube · MrMario2011 Can You Ps2 Games On Ps3
Advanced users often "jailbreak" their consoles using tools like PS3HEN or Custom Firmware to run PS2 ISO files via software emulation on newer models.
You can purchase and download a limited selection of "PS2 Classics" directly from the PlayStation Store. These digital versions are designed to work on any PS3 model. Most compatible models (except the 20GB version) have
Models CECHCxx (60GB PAL) and CECHExx (80GB NTSC). These use a mix of hardware and software, leading to occasional glitches. Playing PS2 Games on Non-Compatible Models
Models CECHAxx (60GB) and CECHBxx (20GB). These contain actual PS2 chips. These tutorials provide detailed walkthroughs for setting up
Look for a sticker on the back or bottom starting with CECH .