For much of my working career, I worked off and on as a graphic artist. In the 70s for an advertising agency and a typesetter. In the 80s and early 90s, for a flexographic printing plate manufacturer. There, about 80% of my job was to replicate already printed material (primarily food packaging) exactly which, of course, meant identifying type. We had a fairly impressive library of type catalogs - everything from hot type specimen books from the 30s and 40s through the latest catalogs of transfer type. I can't even begin to imagine how many hours I spent over those years trying to identify an obscure face. And if we couldn't find it, or it was no longer available, we would have to replicate it by hand, either using photostats from the catalog and assembling them into what we needed, or just plain drawing the letterforms. (Sometimes we'd "fudge" and use a face as nearly identical as possible, and hope the customer didn't complain. I suspect we were more obsessive about it than most of them were, since I don't recall a customer ever complaining that the type didn't match...)

mattkevan's comments are a good primer for quickly identifying one font over another, and are pretty much what we would have done for the "first cut" to disqualify similar faces.

It isn't a job I'd relish today. Back then, in essentially the pre-computer days, you were dealing with thousands of fonts. Today, the choices seem almost infinite.

Like with most things it's just a matter of practice and knowing what to look out for. Same as with birds or leaves or whatever.

Firstly, is it serif, sans-serif, handwritten, monospaced etc? This will narrow it down, but distinguishing between similar fonts, such as Arial/Helvetica, Avenir/Futura, Garamond/Caslon etc. requires looking at individual characters.

Lots of characters don't change much between typefaces and can be hard to identify at small sizes, but others can change a lot so they're the ones to look out for - including the lowercase g, uppercase Q and ampersand &.

Is the g double storey or single storey? Open Sans and Noto are identical except for the g (Open Sans is double storey).

What's the tail on the Q like? Does it cross the circle, is it curved or straight? Open Sans and Droid Sans are identical except that the tail of the Q is curved in Droid Sans and straight in Open Sans. (Also the uppercase I has bars in Droid Sans).

Arial and Helvetica can be told apart by looking at the lowercase t. Arial's t terminal is at and angle whereas Helvetica's is horizontal. Also the terminals of the uppercase S are at an angle whereas Helvetica's is horizontal.

Yes Arial and Helvetica are a special case that I learned to tell apart early on since both are very common. I just use Arial and Helvetica to set the same letters, but in both different colors and set the transparency to 30% or thereabouts. Then I position the letters such that their difference is quite easy to tell.

Thanks for the answer. I guess I just need more practice.

The author is a professional type designer and created some well known fonts - including Proxima Nova and Anonymous Pro. For someone that has spent much of his life looking at and thinking about typefaces, I imagine it's trivial for him to accurately identify hundreds of varieties.

I recommend "The Elements of Typographic Style" by Robert Bringhurst if you'd like to improve your type identification abilities. The appendices have a good, but by no means exhaustive, reference of some of the better known typefaces.

I believe he just has an experience. Some people can identify fonts, others programming languages. Even if you aren't a pro, just search for "guess font from image."