Sample games typically have cool rare features that were cut from the production version to save costs or increase reliability. Often times early production games will also have some of these features still intact. There are also "Proto" games (prototype) which will also have red lamp boards under the pf. Depending on the era, a sample or proto game could also have a rare playfield coating like LS Topcoat or Diamondplate.

DH sample should have a single drop target in the playfield below the warehouse, and possibly other stuff.

I think they used it to test out features before they went into full blown production. They don't pop up often at all. In Dirty Harry there are a few differences between the sample and reg production versions, the biggest one is that the sample has a drop target under the warehouse. I've actually only played a sample so i can't really comment on how much different the game play is because of it.

A true sample game from this era should have red circuit boards.

So what's the difference between samples and prototypes ?

I always heard that samples were free.

Are sample games usually worth more money?

No, that would be a prototype. Samples were made after the prototype run.