Display a blank dark screen (black or dark grey or similar)if there are any dusblobs they will appear as a feint circular blob around 3-4 inch diameter dependent on the size of screen you are viewing.If you de-focus the lens the 3-4"cicles will disappear and a small spec of dust around a few pixels in size will be visible.
Anyhow do not worry about dustblobs generally they are quite easy to dispose of (details elswhere on forum).
Dead pixels are different will normally be seen on blue blank screen.Size is one pixel square, if any are stuck open or closed beware.If any are there, should be no more than 1 or 2 and not in centre area of screen if so don't buy it.
display solid white, black and blue screens. pixels will be small and not necessarily visible on all screens, dust will be bigger and most easily show as green on the white display. Panel alignment will show as coloration around the edges (best again on white screen).