How can this be checked? Is the code public ?
Yes it is. And it's called Wei. there're several sub units on ETH and the smallest one is WEI which is equal to 10^-18 ETH
you can get to know about all the sub units with there value from here, https://etherchain.org/tools/unitConverter
Yes, the code is public:
const ( Wei = 1 GWei = 1e9 Ether = 1e18 )