Designer has spotted a delicious font being used on/offline, but there are no references what is the name of the font. The designer either gets unsatisfactory results from automatic identification services or is unable to provide a digital image of the font.
so its really hard to help :) and if he spotted this font in a design (Picture) we can make some guess and at-least nearby font or something productive answer can be shared.
1. Is there a good checklist that could be followed?
Ans: yes there is, find these point one by one
- Do the characters have serifs?
- What is the position of the upper-case 'Q' tail?
- What style is the '$' (dollar)? and so on see this kind of questions Here
2. Where in the font I should look at?
Ans : Same site which is mentioned above there is simple test showing which kind of font looks like...and where you have to look.

Find fonts by appearance
Find fonts by name
Find fonts by similarity
Find fonts by picture
Find designers and publishers
3. What defines and categorises the font?
Ans: They also want to know
& see Font Categorization
Based on: Font on which it is based on, if any (example: Bitstream Vera)
Ranges: Unicode group ranges (like Latin, Greek, Greek Extended)
OpenType layout tables: (like Latin, Cyrillic, Devanagari)
Font Family: (like Serif, Sans Serif)
Font Styles: (like Roman, Oblique, Bold, BoldOblique)
License: (like shareware, public domain, Open Font License, GPL)
4. How the graphic design professionals/enthusiasts provide answers to font identification questions at here or at other crowdsourcing sites (e.g. Typophile) — is it something else than just memorising a few hundred typefaces?
Ans. There are so many tool to identify font, and there are limited fonts which are used(i mean mostly everyone use common fonts which we see somewhere everyday) so this make sense.
and as all such question got answer on this site, they all are similar in question and they all are similar in answer...there are only few sites who provides such identification answers...
I read this lines in an article "Science sinks millions of dollars into face recognition algorithms, but they’ll never replicate the powerhouse combo of an eye and a brain. Same goes for font recognition—the only thing that can beat a font nerd is a giant horde of nerds. Once again, the Internet provides!
Most font sites have helpful forums. Two that merit special mention are the font lovers at Typophile.com and Flickr’s font identification forum. If they can’t find your font, it might not exist."
I would like to add one thing : you know we are the guys who creates automatic services to identify fonts (or to identify anything ), so you have to be sharp enough to see and recognize a font without help of automatic services...We guys made things automatic based on some concept and programs if you know these concept and ways to solve problems, you can be a FONT GURU...
Hope this will help a bit...