An attacker controls the majority of the hashing power and wants to double spend without arousing suspicion. If they mirror the public chain's activity while generating the private fork and the fork is short (say 10 blocks), can you only detect the double spend by verifying the block hashes haven't changed or noticing the double spend transaction directly?
The short answer is that there is no way to double spend "without arousing suspicion". Presumably, you would be double-spending against someone and that person would notice as soon as you diverted the main chain.
longer answer: As long as you're building your private chain you can double spend everything you have by normally broadcasting to the public chain, knowing that it will be reversed later. There is no way to detect/defend against a 51% attack until it happens, and once it happens the security of the blockchain of Xcoin is obviously compromised and the whole coin/your stash of coins is now worthless, so there was not much point in the double spend anyway.