I'm assuming you want to be able to tell at launch, most of us start yelling pretty loud if its becoming an instamine so just read the thread thoroughly. Normally the dev will call a premine a premine, but look for any odd language setting aside coins for anything other than POS and/or POW rewards.
Easy(-ish) Instamine check for within 24 hours of launch: Assuming you are comfortable downloading the new coin's wallet (suggest virtual machine in case theres hidden virus) sync the blockchain then take the current time subtract the time of launch to tell you how long its been released for mining, then divide that time by the blocks solved to get blocks per minute. Coin announces tell you the target time per blocks, if an incredibly larger amount have been mined than should have the difficulty retarget is not performing as planned and it is turning into an instamine. Few miners can get large amounts of coins that should take hours/days/weeks and the trouble is they are candidates for a dump as they cost much less to mine than the coins in the future emitted at the projected normal rate. Example, 1 coin per block, 1 minute target block time should emit 1440 coins per day, if at the end of the day you see 5000 blocks solved you can tell the retarget did not work and it has been (slightly) instamined. (for what its worth, as more miners join a new coin blocks get solved ahead of schedule while retargets kick in, its not weird to see more solved than planned for to some degree as the coin is running up towards a steady hashrate)
Premine check: most devs are up front about wanting a premine so you can usually tell by the announcement, heck they do deserve to get paid eventually for their work unfortunately with pow coins it can take months for miners to catch up to 1 or 2 % of the coin while one person has held that amount from the start with no guarantee as to what their plans are it can be a dark cloud over a coin. POS coins with a 1-2% premine can be dangerous too though with all the coins emitted there are miners who may easily control many percentages of a coin, it can be dumped but it could be by anyone. in the code you can go to a coins github page and navigate to /src/main.cpp for instance squarebit coin has a 2% premine listed on their announce (opened random announces til i found one with a premine nothing against squarebit) scroll around the main.cpp file until you find " CBlock and CBlockIndex" it will tell you about the rewards at said blockheights. in the case of squarebit we can se 40,000 coins were created in block 1, then first set of rewards is 500 per block, reducing to zero by block 10080 for the POS phase as can be seen here:
https://github.com/Squarebitdata/Sbit/blob/master/src/main.cpp#L971sorry this may be more complicated answer than your looking for. truth is if its was premined or instamined doesnt mean its garbage, and even a perfect launch and no premine can still end up in flames.
