Basically, just look for strong early poke. The factor which distinguishes ad champs from ap ones in power is the fact that ap champs scale best with experience while ad champs scale best with items (gold). This means that an equally-farmed ap champ will typically be more powerful than an ad one in early and mid game, before the ad champ gets items past his core ones. Thus, pick a champ with good poke for harass and farming -- basically, a strong early game presence.
Cait is good at this, as are Talon, Ez, and many others.