Optima is a very famous typeface. It's probably the most famous humanist sans --- these are faces that try to capture the quirks of handwriting or calligraphy. The uppercase letters mimic carved-in-stone Roman letters. Because it has serif-y features (flared terminals in particular) it's notoriously hard to harmonize with other faces. Every designer in the world can spot it on sight.

In general, look at the contrast in weights (where do the lines get thinner and thicker) and the axis of those contrasts (is the "hand" drawing the letter positioned at an angle?). In a Sans, you can also eyeball how "geometric" the letters are; is the "o" a perfect circle, for instance?

Even if you're not a designer (I'm definitely not), it's worth it to buy a copy of Bringhurst's _Elements of Typography_; it's a beautifully written and designed little book.