I would not recommend watching one half hour piece, it's better to compare as much points as possible. I always zap through the movie watching segments of about 2 or 3 minutes, and try to include as much facettes as I can (space shots, movement, desert, close ups of R2, dark sequences, light sequences, smoke, light-sabres). Everytime you compare you see new things, so a full list of what I look at would be way too long. But here are a couple of things.

In the opening shot check out the texture of the moon on the left. Determine how long the Tantive's engines are seperable. Look for grain at the bottom of the stardestroyer, and determine how sharp it appears (lines clearly distinguishable?). Freeze frame on a couple of red laserblasts to check the texture for moire patterns or lines around them. When the stormtroopers come rushing into the Tantive check the texture of the smoke with laserblasts in it. And while watching pieces of the Tantive sequence pay attenton to persons moving fast in front of the camera: are they sharp in motion.
In the opening shot on Tatooine determine how sharp 3PO appears during the zoom in (for example you'll see that on EditDroid his texture is distinguisable very early in the zoom-in). Check out the texture of the sky and sand in the following sequence (3PO walking though the desert). To see how clear the limited vertical resolution stands out, check R2 close ups in the sandcrawler. Look how sharp the shot is where Luke looks at Aunt Beru when she sais the droid must speak Bocce, pay close attention to the moisture vaporator in the middle of the shot. Check out Luke's movement in the sequence where 3PO gets his oil bath, is it sharp or does it "wipe". Check the texture of the desert in the first shot of Luke and 3PO in the speeder (here you'll clearly see one of the few flaws of the EditDroid set). Check out some of the following desert-shots. And pay attention to how sharp the shots in Ben's home are. Then skip all the way through to the Darth/Ben fight to check out the sabres. And of course keep your eyes open for anything that looks suspicious

I would also recommend to sort them from least to best, and watch 'em in that order. Since the differences are so minute sometimes, that way it's much easier to compare. If you haven't done any comparing yet, you could use my list in the "official reviews" thread

Use an as big as possible screen, and a HQ DVD player. Increase the color on the pale transfers (EditDroid and Moth3r), view as much transfers as you can get: I still have all ep IV's discussed in my review, except for Farsight. If I need to post one on a.b.starwars just say so. Have fun and don't forget to post your review.