Vz 52/57's are very interesting, especially in the way they combine various design elements: the M1-like trigger group, annular gas/piston, and folding bayonet.
As mentioned, familiarize yourself with the difference between Vz52 and 52/57 magazines. The former have an extra angle along the bottom of the front edge.
There are typically 5 places where serial numbers are marked:
1) side of buttstock
2) side of receiver
3) bolt
4) under front handguard
5) top of operating rod (this is the sheet metal wrapped around the barrel that pushes against the bolt carrier.
I'm going by memory, but I believe the serial on the bolt is electro-penciled and is much easier to see under strong light, especially if the bolt has been refurbed/reblued. In Canada, it's pretty uncommon to find a completely matching Vz 52/57 --- many of the handguards are mismatched, as well as bolts. The cleaning kit (under the buttplate) is also often missing or incomplete.
A lot of Vz 52/57s were refurbed at some point, which may (not positive) be indicated by the colour of the paint on the front handguard metal and the receiver colour: black is original and grey is refurb.
Also, don't fret if the stock has a green hue; this is pretty typical. The finish will also be a varnish that can sometimes feel a bit gritty.
As has been mentioned, the piston can sometimes get stuck due to corrosion if the rifle was not properly cleaned. The fitting of the piston to the machined/polished part of the barrel has tight tolerances. Even if you don't have the carbon scraper, a piece of stiff paper can be used to get under the piston. Alternatively, there's a nut that you can loosen and slide the piston down so that you can clean the barrel surface. If I recall, there is a second lock nut that preserves the gas setting when you re-tighten the main nut.
Anyways, they're great rifles with an interesting design. Spare Vz 52/57 magazines are hard to come by, so snatch them up when you find them.