I was kind of curious as to whether there was a way to tell the difference between cast and forged lowers.

I've read for instance that most Essential Arms were cast, some Olympic lowers were cast, and some PWA lowers were cast (I'm obviously talking about prebans here).

I know Olympic has pictures on there website that show the difference between their cast and forged lowers. But how about other manufacturers? Can you tell just by looking at them?

I always look at the lettering.  Not the unique stuff like the serial #, but the common stuff like the company name/logo, the safe and fire by the selector, etc.

On a cast part the lettering is a part of the mold, but you can't do good clean inset lettering or finely detailed logos when you cast so the lettering is either raised or it'll be rather coarse type stuff.  But on a forged lower all of that will be roll stamped.  So if everything is roll stamped then it is probably forged.   If you see any raised lettering then you can be sure it is cast.  

There are some other things like looking for cast marks and such.  Also, there can be specific marks such as the 'f' suffux that DPMS adds to the serial numbers of their forged lowers.