Obviously looking up the stamped suffix code is the best way to know what you've actually got in front of you..
But besides looking up castings or recognizing certain suffixes and the loose patterns they went by, here's what I always do..

First is look at the shape of the suffix pad..
Is it a long more rectangular older style or the 75/76'ish and later engines w/ the smaller, squarer suffix pad?.
Knowing this much at least shows what the possibilities of cubes could be and you can discount 305/267's etc if the earlier pad style..

Next is always keep in mind the '69 and earlier engines have a 2 digit suffix and the '70 and up 3 digit Suffixes..
The '70 3 digit suffixes usually start w/ a C or T to denote Car or Truck engines..

Now look at the smaller stamped digits below the build/suffix stamp which is the serial number of the car it was originaly in..
Of course this only works on engines that have the serial # digits stamped on the pad but most do..
Looking at this number count backwards the 8 digits and that's the model year..

Quite a bit to know this far in and often enough to say yes or no to what's in front of you without having to write anything down or look any #'s up yet!.

Heads and other bolt on parts are good clues too but can be changed so doing the above then looking up Suffixes or Castings if it's worth going further..

Also keep in mind The early 305's had 305 cast into the bellhousing flange of the block and you could see it if you peer down behind the distributor too..