How to diagnose screen artifacts - monitor or video card?

Over the last few days I've begun seeing violet or very light purple
artifacts on my monitor (Mag 19" LCD on a Mac Pro w/NVIDIA GeForce 7300
GT video card & OS 10.5.1). On some desktop pictures it will manifest
as groups or bands of discolored pixels in random spots. On others,
nothing at all. Today I started seeing horizontal lines running through
the title bars of some applications and preference panes, but again not
all. It doesn't show on the grey boot screen, but does show up at the
login screen

Googling yielded some complaints in gamers forums about various video
card problems but nothing that seemed to match this.

Is there a diagnostic tool that might help determine whether it's the
video card or monitor at fault? I don't have a spare monitor; I bought
this setup because the old stuff was burglarized.

Thanks in advance.