Identifying marbles is a fun hobby, and can take a lifetime to know all the different kinds available. Marbles have evolved over the years from a game for children to play to a collectible form of glass and art glass. Many marbles are not glass, though, and they are also interesting. Anything in the shape of an orb is a marble to me, and holds my interest with the artistic merit of this art form.
Identify marbles by art glass.
Look for art glass marbles. There are many that were handmade in Germany up until about World War II, and they have latticino canes in a swirl. There are many different kinds of latticino swirls, and they are identified by the center. Some are called naked core, some solid core, and latticino core.
Identify opaque glass.
Find opaque glass marbles. Many of the good older marbles are opaque, not transparent, and lighting does not reveal the center. These marbles are glass, not stone.
Look for crockery or stone marbles.
See if you can find marbles that are not glass. Bennington marbles are a crockery, and many marbles are made of stone.
Identify glass marbles by the swirl.
Find corkscrews. A corkscrew has an opaque swirl that runs completely around the marble from top to bottom. There are transparent and opaque swirls, too.
Identify the flame marbles.
Know the flame design in marbles. Flames are usually older and are collectible marbles.
Identify the unusual.
Find some collectible marbles that have less-common names. A calligraphy marble looks like it has squiggles of writing.
Identify the commons and new marbles.
Recognize the run-of-the-mill marbles that have no significant value. These are usually not very attractive, newer than the 1960s, machine made, produced in mass, and often imported. They often have fine bubbles in the glass, and the quality is poor.
Identify marbles by comparison.
Find marbles that you think might have value, and compare them to pictures on the internet. Like any glass or art glass, condition and rarity are key to the value of marbles.
Check condition of a marble, too.
While identifying a marble, look with a magnifying glass to see if it has "eyelashes" or little dings. You will be surprised how much damage the older used marbles often have. Since condition is related to value, it is helpful to identify the condition along with the kind of marbles.