You can usually spot a premine quickly by looking at the code. There are a few coins who had a hidden premine, but that can also be found by digging deeper into the code. Another way a coin can be premised is through an instamine (I know, it's technically not a premine, but in reality it is). On those, you need to watch the mining activity and check the explorer for who mined those blocks. Also beware of coins that are on block 1000 upon release. Those 1000 blocks are a premine. There are all kinds of ways to pre/instamine a coin. The key is to pay attention to what's hoping on and check the code.
alright i just read about the instamine when you said it, man the dev is a genius. anyways, moving ...