Complementary to the setmetatable method, you can retrieve the meta table of a table using the getmetatable method. getmetatable takes only one argument, a table. If the table provided has a meta table, the meta table will be returned, otherwise getmetatable will return nil:

x = {}
y = {}
z = {}

setmetatable(y, z)

print (getmetatable(x))
print (getmetatable(y))

This will print nil for x, as it has no meta table, and some large, random number for y. This large, random-looking number is the unique ID for table z.