There's probably nothing that can be diagnosed, as such: if the card is working then... it's likely Ok.
Generally failures manifest as corrupted or no display, visual glitches, insane flickering or other artifacts.
nVidia has a system utility available, which is the nearest I've seen to diagnostics. It's mainly concerned with overclocking and performance tweaking, which isn't what you're after - but amongst that stuff is temperature monitoring. (I think this is probably also available from stand-alone, generic system utilities - Googling with nVidia GPU monitor may be useful).
Also, there's a chance the system utility doesn't support the card you have.
http://www.nvidia.com/object/nvidia-sys ... river.html
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Giraffe: a ruminant with a view.