Hey, David -
I'll agree with Ethan that a sample of work is the way to go. As for the elements I'd look for in the screencast:
1. Personality / character. Does the caster have a charismatic and warm quality that makes it easy to keep watching? This one is the hardest to develop and the toughest to find, in my opinion.
2. Pace / Voice. How does the caster sound? Again, it goes back to easy to listen to. Do they read too fast, too slow, or just right? There's lots of overlap with personality / character here. How smooth is the cast? Does it seem natural or scripted?
3. Clarity of explanation. This is a reflection of both planning and ability to improvise. Are the ideas scaffolded well? Does the caster use just the right dose of enhancement of the procedure to bring it home?
I'd ask for 2 different examples. 1) Deliver as if your audience doesn't have audio capabilities and 2) Deliver as if your audience can hear you. A good screencaster will have no trouble with either of these challenges. And in each case, I believe the best casters will make you feel like the screencast was made just for you.