When I bought my Playstation 2 a few years ago, it was already modded by a previous owner. The guy had told me that a DMS4 was installed, but I later discovered (much to my dismay) that that was not the case when I wasn't able to install ToxicOS.
I <3 my PS2, I have a Network Adapter and a 160GB hard drive installed, and I can play any number of games off the hard drive using a HDLoader 0.8c CD.
But I'd still like to find out exactly what type of modchip I have installed in my PS2. Is there any way to do that? I'd strongly prefer not to open up the console at all and I'm certainly not going to disassemble working hardware just to satiate my curiosity, but if there's an easier way to find out which modchip I have, I'd love to know about it.