$ffffffffff NVIDIA VGA BIOS information
$ffffffffff ---------------------------------------------------
$1100000000 Title           : GeForce 6800 VGA BIOS HDN1
$1100000002 Version         : 5.40.02.12
$1100000001 BMP version     : 5.28
$1100000100 BIT version     : 1.00
$1100010000 Perf. level 0   : 325MHz/350MHz/1.20V
$1100020000 VID bitmask     : 00000011b
$1100020100 Voltage level 0 : 1.10V, VID 00000000b
$1100020101 Voltage level 1 : 1.20V, VID 00000001b
$1100020102 Voltage level 2 : 1.40V, VID 00000010b
$1100000003 GPU units mask  : 16x1,6vp

Q: How can I see if my GeForce 6800 is softmoddable or not? Does the moddability depend on display adapter manufacturer?

A: No, it doesn't depend on brand. The only factor defining softmoddability success is an amount of physically masked pixel/vertex units in GPU.

To see if your adapter is softmoddable or not, you may use RivaTuner's graphics subsystem diagnostic report module and look at "GPU units mask" line in "NVIDIA VGA BIOS information" report category. If you'll see there something different from 16x1,6vp (e.g. 12x1,6vp) - it means that graphics adapter manufacturer tried to lock some graphics processor's pixel/vertex units in software. Otherwise VGA BIOS doesn't mask any units, your graphics processor's configuration is defined by amount physically masked units and you may forget about softmodding.