In case of altcoins which are based on bitcoin code, it's not very difficult.
You don't have to review a ton of code in this case. You need to follow a few simple steps.
1) Find a version of bitcoin which altcoin is based on.
2) Download a bitcoin source code for this version.
3) Use diff tool for comparison.

You can simplify it a little bit further.
For instance: a vast majority of pos coins based on Peercoin code.
As a result if you need to review any of those forks, you do above mentioned steps on Peercoin code instead of bitcoin code.

P.S.: Pay a special attention to GetBlockSubsidy function. Usually all premines are defined there.