How do I tell a fair coin from a premined one?

Put them side by side on a few axes. Check whether the genesis block is clean or credits large founder balances. Reconcile each coin's circulating supply against mined blocks; a gap points to a premine. Read each project's documentation for disclosed allocations, treasuries, or token sales. Look at early address concentration, since premined coins often show a few dominant wallets. Finally, check whether new coins can only come from mining or whether a team keeps a mint function. A fair emission like Malairte passes each test; a premined coin fails one or more, usually obviously once laid out.