the very first thing you see when you boot your computer should be the video card BIOS. it will appear for about 2 seconds then your computers BIOS will boot. look and see if you see M64 anywhere on that and if not it is probably a regular TNT
The specifications say its a 128bit chip. All TNT2 chips have an internal 128bit datapath. The TNT2 M64 however has a 64bit memory bus as compared to 128bit for regular/Ultra TNT2s.
However, it does look like a regular TNT2 as there's no mention of M64 anywhere.