Rough weights for medium lines of cymbal are 2500-2800g for 22˝, 2050-2300g for 20˝ and 1650-1850g for 18˝.
If yours is lighter than that you've got a clue that it's an older Zildjian. Manufacturers often change the sizes of cymbal holes.
Once the paint gets rubbed off from cleaning it's easy to mistake a hi hat cymbal for a small crash or even a splash cymbal.
Old cymbal catalogs often have the sizes of the cymbals for a given year but if your hi hat is the same size as a crash for that year you have to find another way to identify the cymbal.
These cymbals aren't vintage Zildjian cymbals but have easy to spot visual examples of the cymbal characteristics above.